[ios, macos] Reorganized custom function documentation

Reorganized the custom function documentation as a series of sections with headers and definition lists instead of a monolithic table. Copyedited various sections for accuracy. Linked references to custom functions. Added format string examples for all custom functions.

1 files changed, 411 insertions, 218 deletions

diff --git a/platform/darwin/docs/guides/Predicates and Expressions.md b/platform/darwin/docs/guides/Predicates and Expressions.md
index fff0992454..89e4983b9c 100644
--- a/platform/darwin/docs/guides/Predicates and Expressions.md
+++ b/platform/darwin/docs/guides/Predicates and Expressions.md
@@ -136,7 +136,7 @@ NSExpression(forFunction: "lowercase:",
              arguments: [NSExpression(forKeyPath: "ISO 3166-1:2006")])
 ```
 
-### Predefined functions
+### Functions
 
 Of the
 [functions predefined by the `+[NSExpression expressionForFunction:arguments:]` method](https://developer.apple.com/documentation/foundation/nsexpression/1413747-init#discussion),
@@ -169,7 +169,10 @@ Initializer parameter | Format string syntax
 `length:`             | `length('Wapakoneta')`
 `castObject:toType:`  | `CAST(ele, 'NSString')`<br>`CAST(ele, 'NSNumber')`
 
-The following predefined functions are not supported:
+A number of [Mapbox-specific functions](#mapbox-specific-funcions) are also
+available.
+
+The following predefined functions are **not** supported:
 
 Initializer parameter | Format string syntax
 ----------------------|---------------------
@@ -187,228 +190,13 @@ Initializer parameter | Format string syntax
 `onesComplement:`     | `onesComplement(255)`
 `distanceToLocation:fromLocation:` | `distanceToLocation:fromLocation:(there, here)`
 
-### Mapbox-specific functions
-
-For compatibility with the Mapbox Style Specification, the following functions
-are defined by this SDK. When setting a style layer property, you can call these
-functions just like the predefined functions above, using either the
-`+[NSExpression expressionForFunction:arguments:]` method or a convenient format
-string syntax:
-
-Initializer parameter | Format string syntax | Description
-----------------------|----------------------|------------
-`mgl_does:have:` | `mgl_does:have:(SELF, 'key')` or `mgl_does:have:(%@, 'key')` | Returns a Boolean value indicating whether the dictionary has a value for the key or whether the evaluated object (`SELF`) has a value for the feature attribute. Compared to the `mgl_has:` custom function, that function’s target is instead passed in as the first argument to this function. Both functions are equivalent to the syntax `key != NIL` or `%@[key] != NIL` but can be used outside of a predicate.
-`mgl_interpolate:withCurveType:parameters:stops:` | `mgl_interpolate:withCurveType:parameters:stops:(x, 'linear', nil, %@)` | Produces continuous, smooth results by interpolating between pairs of input and output values (“stops”). Compared to the `mgl_interpolateWithCurveType:parameters:stops:` custom function, the input expression (that function’s target) is instead passed in as the first argument to this function.
-`mgl_step:from:stops:` | `mgl_step:from:stops:(x, 11, %@)` | Produces discrete, stepped results by evaluating a piecewise-constant function defined by pairs of input and output values ("stops"). Compared to the `mgl_stepWithMinimum:stops:` custom function, the input expression (that function’s target) is instead passed in as the first argument to this function.
-`mgl_join:` | `mgl_join({'Old', 'MacDonald'})` | Returns the result of concatenating together all the elements of an array in order. Compared to the `stringByAppendingString:` custom function, this function takes only one argument, which is an aggregate expression containing the strings to concatenate.
-`mgl_round:` | `mgl_round(1.5)` | Returns the number rounded to the nearest integer. If the number is halfway between two integers, this function rounds it away from zero.
-`mgl_coalesce:` | `mgl_coalesce({x, y, z})` | Returns the first non-`nil` value from an array of expressions.
-`MGL_LET` | `MGL_LET('age', uppercase('old'), 'name', uppercase('MacDonald'), mgl_join({$age, $name}))` | Any number of variable names interspersed with their assigned `NSExpression` values, followed by an `NSExpression` that may contain references to those variables. Compared to the `mgl_expressionWithContext:` custom function, this function takes the variable names and values inline before the expression that contains references to those variables.
-`MGL_MATCH` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')` | Evaluates the first expression and returns the value that matches the initial condition. After the first expression condition a pair of matching/return value should be added and a default value.
-`MGL_IF` | `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)` | Returns the first value that meets the condition otherwise a default value. The expression conditions should be added in pairs of conditional/return value. Unlike `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or the `TERNARY()` syntax, this function can accept multiple “if else” conditions and is supported on iOS 8._x_ and macOS 10.10._x_; however, each conditional passed into this function must be wrapped in a constant expression.
-
-The following custom functions are also available with the
-`+[NSExpression expressionForFunction:selectorName:arguments:]` method or the
-`FUNCTION()` format string syntax:
-
-<table>
-<thead>
-<tr><th>Selector name</th><th>Target</th><th>Arguments</th><th>Returns</th></tr>
-</thead>
-<tbody>
-<tr>
-   <td><code>boolValue</code></td>
-   <td>
-      An <code>NSExpression</code> that evaluates to a number or string.
-   </td>
-   <td></td>
-   <td>
-      A Boolean representation of the target: <code>FALSE</code> when then input is an
-      empty string, 0, <code>FALSE</code>, <code>NIL</code>, or <code>NaN</code>, otherwise <code>TRUE</code>.
-   </td>
-</tr>
-<tr>
-   <td><code>mgl_has:</code></td>
-   <td>
-      An <code>NSExpression</code> that evaluates to an <code>NSDictionary</code> or the evaluated object (<code>SELF</code>).
-   </td>
-   <td>
-      An <code>NSExpression</code> that evaluates to an <code>NSString</code> representing the key to look up in the dictionary or the feature attribute to look up in the evaluated object (see <code>MGLFeature.attributes</code>).
-   </td>
-   <td>
-      <code>true</code> if the dictionary has a value for the key or if the evaluated object has a value for the feature attribute.
-   </td>
-<tr>
-   <td><code>mgl_expressionWithContext:</code></td>
-   <td>
-      An <code>NSExpression</code> that may contain references to the variables defined in
-      the context dictionary.
-   </td>
-   <td>
-      An <code>NSDictionary</code> with <code>NSString</code>s as keys and <code>NSExpression</code>s as values.
-      Each key is a variable name and each value is the variable’s value within
-      the target expression.
-   </td>
-   <td>
-      The target expression with variable subexpressions replaced with the
-      values defined in the context dictionary.
-   </td>
-</tr>
-<tr>
-   <td><code>mgl_interpolateWithCurveType:parameters:stops:</code></td>
-   <td>
-      An <code>NSExpression</code> that evaluates to a number and contains a variable or
-      key path expression.
-   </td>
-   <td>
-      The first argument is one of the following strings denoting curve types:
-      <code>linear</code>, <code>exponential</code>, or <code>cubic-bezier</code>.
-      
-      The second argument is an expression providing parameters for the curve:
-      
-      <ul>
-         <li>If the curve type is <code>linear</code>, the argument is <code>NIL</code>.</li>
-         <li>
-            If the curve type is <code>exponential</code>, the argument is an expression
-            that evaluates to a number, specifying the base of the exponential
-            interpolation.
-         </li>
-         <li>
-            If the curve type is <code>cubic-bezier</code>, the argument is an array or
-            aggregate expression containing four expressions, each evaluating to
-            a number. The four numbers are control points for the cubic Bézier
-            curve.
-         </li>
-      </ul>
-      
-      The third argument is an <code>NSDictionary</code> object representing the
-      interpolation’s stops, with numeric zoom levels as keys and expressions as
-      values.
-   </td>
-   <td>
-      A value interpolated along the continuous mathematical function defined by
-      the arguments, with the target as the input to the function.
-   </td>
-</tr>
-<tr>
-   <td>
-      <code>mgl_numberWithFallbackValues:</code>,
-      <code>doubleValue</code>,
-      <code>floatValue</code>,
-      <code>decimalValue</code>
-   </td>
-   <td>
-      An <code>NSExpression</code> that evaluates to a Boolean value, number, or string.
-   </td>
-   <td>
-      Zero or more <code>NSExpression</code>s, each evaluating to a Boolean value or
-      string.
-   </td>
-   <td>
-      A numeric representation of the target:
-      <ul>
-         <li>If the target is <code>NIL</code> or <code>FALSE</code>, the result is 0.</li>
-         <li>If the target is true, the result is 1.</li>
-         <li>
-            If the target is a string, it is converted to a number as specified
-            by the
-            “<a href="https://tc39.github.io/ecma262/#sec-tonumber-applied-to-the-string-type">ToNumber Applied to the String Type</a>”
-            algorithm of the ECMAScript Language Specification.
-         </li>
-         <li>
-            If multiple values are provided, each one is evaluated in order
-            until the first successful conversion is obtained.
-         </li>
-      </ul>
-   </td>
-</tr>
-<tr>
-   <td><code>mgl_stepWithMinimum:stops:</code></td>
-   <td>
-      An <code>NSExpression</code> that evaluates to a number and contains a variable or
-      key path expression.
-   </td>
-   <td>
-      The first argument is an expression that evaluates to a number, specifying
-      the minimum value in case the target is less than any of the stops in the
-      second argument.
-      
-      The second argument is an <code>NSDictionary</code> object representing the
-      interpolation’s stops, with numeric zoom levels as keys and expressions as
-      values.
-   </td>
-   <td>
-      The output value of the stop whose key is just less than the evaluated
-      target, or the minimum value if the target is less than the least of the
-      stops’ keys.
-   </td>
-</tr>
-<tr>
-   <td><code>stringByAppendingString:</code></td>
-   <td>
-      An <code>NSExpression</code> that evaluates to a string.
-   </td>
-   <td>
-      One or more <code>NSExpression</code>s, each evaluating to a string.
-   </td>
-   <td>
-      The target string with each of the argument strings appended in order.
-   </td>
-</tr>
-<tr>
-   <td><code>stringValue</code></td>
-   <td>
-      An <code>NSExpression</code> that evaluates to a Boolean value, number, or string.
-   </td>
-   <td></td>
-   <td>
-      A string representation of the target:
-      <ul>
-         <li>If the target is <code>NIL</code>, the result is the string <code>null</code>.</li>
-         <li>
-            If the target is a Boolean value, the result is the string <code>true</code> or
-            <code>false</code>.
-         </li>
-         <li>
-            If the target is a number, it is converted to a string as specified
-            by the
-            “<a href="https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type">NumberToString</a>”
-            algorithm of the ECMAScript Language Specification.
-         </li>
-         <li>
-            If the target is a color, it is converted to a string of the form
-            <code>rgba(r,g,b,a)</code>, where <var>r</var>, <var>g</var>, and <var>b</var>
-            are numerals ranging from 0 to 255 and <var>a</var> ranges from 0 to
-            1.
-         </li>
-         <li>
-            Otherwise, the target is converted to a string in the format
-            specified by the
-            <a href="https://tc39.github.io/ecma262/#sec-json.stringify"><code>JSON.stringify()</code></a>
-            function of the ECMAScript Language Specification.
-         </li>
-      </ul>
-   </td>
-</tr>
-</tbody>
-</table>
-
-Some of these functions are defined as methods on their respective target
-classes, but you should not call them directly outside the context of an
-expression, because the result may differ from the evaluated expression’s result
-or may result in undefined behavior.
-
-The Mapbox Style Specification defines some operators for which no custom
-function is available. To use these operators in an `NSExpression`, call the
-`MGL_FUNCTION()` function with the same arguments that the operator expects.
-
 ### Conditionals
 
 Conditionals are supported via the built-in
 `+[NSExpression expressionForConditional:trueExpression:falseExpression:]`
 method and `TERNARY()` operator. If you need to express multiple cases
 (“else-if”), you can either nest a conditional within a conditional or use the
-`MGL_IF()` or `MGL_MATCH()` function.
+[`MGL_IF()`](#code-mgl_if-code) or [`MGL_MATCH()`](#code-mgl_match-code) function.
 
 ### Aggregates
 
@@ -492,3 +280,408 @@ of a [Mapbox-specific function](#mapbox-specific-functions) that takes an
 ```swift
 NSExpression(format: "MGL_LET(floorCount, 2, $floorCount + 1)")
 ```
+
+## Mapbox-specific functions
+
+For compatibility with the Mapbox Style Specification, the following functions
+are defined by this SDK. When setting a style layer property, you can call these
+functions just like the predefined functions above, using either the
+`+[NSExpression expressionForFunction:arguments:]` method or a convenient format
+string syntax:
+
+### `mgl_does:have:`
+
+<dl>
+<dt>Selector:</dt>
+<dd>`mgl_does:have:`</dd>
+<dt>Format string syntax:</dt>
+<dd><code>mgl_does:have:(SELF, 'key')</code> or <code>mgl_does:have:(%@, 'key')</code></dd>
+</dl>
+
+Returns a Boolean value indicating whether the dictionary has a value for the
+key or whether the evaluated object (`SELF`) has a value for the feature
+attribute. Compared to the [`mgl_has:`](#code-mgl_has-code) custom function,
+that function’s target is instead passed in as the first argument to this
+function. Both functions are equivalent to the syntax `key != NIL` or
+`%@[key] != NIL` but can be used outside of a predicate.
+
+### `mgl_interpolate:withCurveType:parameters:stops:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_interpolate:withCurveType:parameters:stops:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>mgl_interpolate:withCurveType:parameters:stops:(x, 'linear', nil, %@)</code></dd>
+</dl>
+
+Produces continuous, smooth results by interpolating between pairs of input and
+output values (“stops”). Compared to the
+[`mgl_interpolateWithCurveType:parameters:stops:`](#code-mgl_interpolatewithcurvetype-parameters-stops-code)
+custom function, the input expression (that function’s target) is instead passed
+in as the first argument to this function.
+
+### `mgl_step:from:stops:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_step:from:stops:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>mgl_step:from:stops:(x, 11, %@)</code></dd>
+</dl>
+
+Produces discrete, stepped results by evaluating a piecewise-constant function
+defined by pairs of input and output values ("stops"). Compared to the
+[`mgl_stepWithMinimum:stops:`](#code-mgl_stepwithminimum-stops-code) custom
+function, the input expression (that function’s target) is instead passed in as
+the first argument to this function.
+
+### `mgl_join:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_join:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>mgl_join({'Old', 'MacDonald'})</code></dd>
+</dl>
+
+Returns the result of concatenating together all the elements of an array in
+order. Compared to the
+[`stringByAppendingString:`](#code-stringbyappendingstring-code) custom
+function, this function takes only one argument, which is an aggregate
+expression containing the strings to concatenate.
+
+### `mgl_round:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_round:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>mgl_round(1.5)</code></dd>
+</dl>
+
+Returns the number rounded to the nearest integer. If the number is halfway
+between two integers, this function rounds it away from zero.
+
+### `mgl_coalesce:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_coalesce:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>mgl_coalesce({x, y, z})</code></dd>
+</dl>
+
+Returns the first non-`nil` value from an array of expressions.
+
+### `MGL_LET`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>MGL_LET:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>MGL_LET('age', uppercase('old'), 'name', uppercase('MacDonald'), mgl_join({$age, $name}))</code></dd>
+<dt>Arguments:</dt>
+<dd>
+   Any number of variable names interspersed with their assigned
+   <code>NSExpression</code> values, followed by an <code>NSExpression</code>
+   that may contain references to those variables.
+</dd>
+</dl>
+
+Returns the result of evaluating an expression with the given variable values.
+Compared to the
+[`mgl_expressionWithContext:`](#code-mgl_expressionwithcontext-code) custom
+function, this function takes the variable names and values inline before the
+expression that contains references to those variables.
+
+### `MGL_MATCH`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>MGL_MATCH:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`</code></dd>
+<dt>Arguments:</dt>
+<dd>
+   An input expression, then any number of argument pairs, followed by a default
+   expression. Each argument pair consists of a constant value followed by an
+   expression to produce as a result of matching that constant value.
+</dd>
+</dl>
+
+Returns the result of matching the input expression against the given constant
+values.
+
+### `MGL_IF`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>MGL_IF:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`</code></dd>
+<dt>Arguments:</dt>
+<dd>
+   Alternating <code>NSPredicate</code> conditionals and resulting expressions,
+   followed by a default expression.
+</dd>
+</dl>
+
+Returns the first expression that meets the condition; otherwise, the default
+value. Unlike
+`+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or
+the `TERNARY()` syntax, this function can accept multiple “if else” conditions
+and is supported on iOS 8._x_ and macOS 10.10._x_; however, each conditional
+passed into this function must be wrapped in a constant expression.
+
+### `MGL_FUNCTION`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>MGL_FUNCTION:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`MGL_FUNCTION('typeof', mystery)`</code></dd>
+<dt>Arguments:</dt>
+<dd>
+   Any arguments required by the expression operator, as defined in the
+   [Mapbox Style Specification](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions).
+</dd>
+</dl>
+
+## Custom functions
+
+The following custom functions are also available with the
+`+[NSExpression expressionForFunction:selectorName:arguments:]` method or the
+`FUNCTION()` format string syntax.
+
+Some of these functions are defined as methods on their respective target
+classes, but you should not call them directly outside the context of an
+expression, because the result may differ from the evaluated expression’s result
+or may result in undefined behavior.
+
+The Mapbox Style Specification defines some operators for which no custom
+function is available. To use these operators in an `NSExpression`, call the
+[`MGL_FUNCTION()`](#code-mgl_function-code) function with the same arguments
+that the operator expects.
+
+### `boolValue`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>boolValue</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`FUNCTION(1, 'boolValue')`</code></dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to a number or string.
+</dd>
+<dt>Arguments:</dt>
+<dd>None.</dd>
+</dl>
+
+A Boolean representation of the target: <code>FALSE</code> when then input is an
+empty string, 0, <code>FALSE</code>, <code>NIL</code>, or <code>NaN</code>,
+otherwise <code>TRUE</code>.
+
+### `mgl_has:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_has:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`FUNCTION($featureProperties, 'mgl_has:', '🧀🍔')`</code></dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to an <code>NSDictionary</code>
+   or the evaluated object (<code>SELF</code>).
+</dd>
+<dt>Arguments:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to an <code>NSString</code>
+   representing the key to look up in the dictionary or the feature attribute to
+   look up in the evaluated object (see <code>MGLFeature.attributes</code>).
+</dd>
+</dl>
+
+<code>true</code> if the dictionary has a value for the key or if the evaluated
+object has a value for the feature attribute.
+
+### `mgl_expressionWithContext:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_expressionWithContext:</code></dd>
+<dt>Format string syntax:</dt>
+<dd>
+   <code>`FUNCTION($ios + $macos, 'mgl_expressionWithContext:', %@)`</code> with
+   a dictionary containing `ios` and `macos` keys
+</dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that may contain references to the variables
+   defined in the context dictionary.
+</dd>
+<dt>Arguments:</dt>
+<dd>
+   An <code>NSDictionary</code> with <code>NSString</code>s as keys and
+   <code>NSExpression</code>s as values. Each key is a variable name and each
+   value is the variable’s value within the target expression.
+</dd>
+</dl>
+
+The target expression with variable subexpressions replaced with the values
+defined in the context dictionary.
+
+### `mgl_interpolateWithCurveType:parameters:stops:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_interpolateWithCurveType:parameters:stops:</code></dd>
+<dt>Format string syntax:</dt>
+<dd>
+   <code>`FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', NIL, %@)`</code>
+   with a dictionary containing zoom levels as keys
+</dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to a number and contains a
+   variable or key path expression.
+</dd>
+<dt>Arguments:</dt>
+<dd>
+   The first argument is one of the following strings denoting curve types:
+   <code>linear</code>, <code>exponential</code>, or <code>cubic-bezier</code>.
+   
+   The second argument is an expression providing parameters for the curve:
+   
+   <ul>
+      <li>If the curve type is <code>linear</code>, the argument is <code>NIL</code>.</li>
+      <li>
+         If the curve type is <code>exponential</code>, the argument is an
+         expression that evaluates to a number, specifying the base of the
+         exponential interpolation.
+      </li>
+      <li>
+         If the curve type is <code>cubic-bezier</code>, the argument is an
+         array or aggregate expression containing four expressions, each
+         evaluating to a number. The four numbers are control points for the
+         cubic Bézier curve.
+      </li>
+   </ul>
+   
+   The third argument is an <code>NSDictionary</code> object representing the
+   interpolation’s stops, with numeric zoom levels as keys and expressions as
+   values.
+</dd>
+</dl>
+
+A value interpolated along the continuous mathematical function defined by the
+arguments, with the target as the input to the function.
+
+### `mgl_numberWithFallbackValues:`
+
+<dl>
+<dt>Selector:</dt>
+<dd>
+   <code>mgl_numberWithFallbackValues:</code>,
+   <code>doubleValue</code>,
+   <code>floatValue</code>, or
+   <code>decimalValue</code>
+</dd>
+<dt>Format string syntax:</dt>
+<dd><code>`FUNCTION(ele, 'mgl_numberWithFallbackValues:', 0)`</code></dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to a Boolean value, number, or
+   string.
+</dd>
+<dt>Arguments:</dt>
+<dd>
+   Zero or more <code>NSExpression</code>s, each evaluating to a Boolean value
+   or string.
+</dd>
+</dl>
+
+A numeric representation of the target:
+
+* If the target is <code>NIL</code> or <code>FALSE</code>, the result is 0.
+* If the target is true, the result is 1.
+  * If the target is a string, it is converted to a number as specified by the
+    “[ToNumber Applied to the String Type](https://tc39.github.io/ecma262/#sec-tonumber-applied-to-the-string-type)”
+    algorithm of the ECMAScript Language Specification.
+  * If multiple values are provided, each one is evaluated in order until the
+    first successful conversion is obtained.
+
+### `mgl_stepWithMinimum:stops:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>mgl_stepWithMinimum:stops:</code></dd>
+<dt>Format string syntax:</dt>
+<dd>
+   <code>`FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', 0, %@)`</code> with
+   a dictionary with zoom levels as keys
+</dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to a number and contains a
+   variable or key path expression.
+</dd>
+<dt>Arguments:</dt>
+<dd>
+   The first argument is an expression that evaluates to a number, specifying
+   the minimum value in case the target is less than any of the stops in the
+   second argument.
+   
+   The second argument is an <code>NSDictionary</code> object representing the
+   interpolation’s stops, with numeric zoom levels as keys and expressions as
+   values.
+</dd>
+</dl>
+
+The output value of the stop whose key is just less than the evaluated target,
+or the minimum value if the target is less than the least of the stops’ keys.
+
+### `stringByAppendingString:`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>stringByAppendingString:</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`FUNCTION('Old', 'stringByAppendingString:', 'MacDonald')`</code></dd>
+<dt>Target:</dt>
+<dd>An <code>NSExpression</code> that evaluates to a string.</dd>
+<dt>Arguments:</dt>
+<dd>One or more <code>NSExpression</code>s, each evaluating to a string.</dd>
+</dl>
+
+The target string with each of the argument strings appended in order.
+
+### `stringValue`
+
+<dl>
+<dt>Selector:</dt>
+<dd><code>stringValue</code></dd>
+<dt>Format string syntax:</dt>
+<dd><code>`FUNCTION(ele, 'stringValue')`</code></dd>
+<dt>Target:</dt>
+<dd>
+   An <code>NSExpression</code> that evaluates to a Boolean value, number, or
+   string.
+</dd>
+<dt>Arguments:</dt>
+<dd>None.</dd>
+</dl>
+
+A string representation of the target:
+
+* If the target is <code>NIL</code>, the result is the empty string.
+* If the target is a Boolean value, the result is the string `true` or `false`.
+* If the target is a number, it is converted to a string as specified by the
+  “[NumberToString](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type)”
+  algorithm of the ECMAScript Language Specification.
+* If the target is a color, it is converted to a string of the form
+  `rgba(r,g,b,a)`, where <var>r</var>, <var>g</var>, and <var>b</var> are
+  numerals ranging from 0 to 255 and <var>a</var> ranges from 0 to 1.
+* Otherwise, the target is converted to a string in the format specified by the
+  [`JSON.stringify()`](https://tc39.github.io/ecma262/#sec-json.stringify)
+  function of the ECMAScript Language Specification.