---input---
   ## a comment

\## not a comment

#if foo
  ${bar}
#endif

The answer is:
#choose
  #when 0 == 1
    0
  #end
  #when 1 == 1
    1
  #end
  #otherwise
    2
  #end
#end -- comment about choose

#for item in items
  * ${item}
#end

#def greeting(name)
  Hello, ${name}!
#end
${greeting('world')}

#with y=7; z=x+10
  $x $y $z
#end

---tokens---
'   '         Text
'## a comment' Comment
'\n'          Other

'\n'          Other

'\\'          Other
'#'           Other
'#'           Other
' '           Other
'not'         Other
' '           Other
'a'           Other
' '           Other
'comment'     Other
'\n'          Other

'\n'          Text

'#'           Comment.Preproc
'if'          Keyword
' '           Text
'foo'         Name
'\n'          Other

' '           Other
' '           Other
'${'          Comment.Preproc
'bar'         Name
'}'           Comment.Preproc
'\n'          Other

'#'           Comment.Preproc
'endif'       Keyword
'\n'          Other

'\n'          Other

'The'         Other
' '           Other
'answer'      Other
' '           Other
'is:'         Other
'\n'          Other

'#'           Comment.Preproc
'choose'      Keyword
'\n'          Other

'  '          Text
'#'           Comment.Preproc
'when'        Keyword
' '           Text
'0'           Literal.Number.Integer
' '           Text
'=='          Operator
' '           Text
'1'           Literal.Number.Integer
'\n'          Other

' '           Other
' '           Other
' '           Other
' '           Other
'0'           Other
'\n'          Other

'  '          Text
'#'           Comment.Preproc
'end'         Keyword
'\n'          Other

'  '          Text
'#'           Comment.Preproc
'when'        Keyword
' '           Text
'1'           Literal.Number.Integer
' '           Text
'=='          Operator
' '           Text
'1'           Literal.Number.Integer
'\n'          Other

' '           Other
' '           Other
' '           Other
' '           Other
'1'           Other
'\n'          Other

'  '          Text
'#'           Comment.Preproc
'end'         Keyword
'\n'          Other

'  '          Text
'#'           Comment.Preproc
'otherwise'   Keyword
'\n'          Other

' '           Other
' '           Other
' '           Other
' '           Other
'2'           Other
'\n'          Other

'  '          Text
'#'           Comment.Preproc
'end'         Keyword
'\n'          Other

'#'           Comment.Preproc
'end'         Keyword
' '           Text
'-- comment about choose' Comment
'\n'          Other

'\n'          Text

'#'           Comment.Preproc
'for'         Keyword
' '           Text
'item'        Name
' '           Text
'in'          Operator.Word
' '           Text
'items'       Name
'\n'          Other

' '           Other
' '           Other
'*'           Other
' '           Other
'${'          Comment.Preproc
'item'        Name
'}'           Comment.Preproc
'\n'          Other

'#'           Comment.Preproc
'end'         Keyword
'\n'          Other

'\n'          Text

'#'           Comment.Preproc
'def'         Keyword
' '           Text
'greeting'    Name.Function
'('           Punctuation
'name'        Name
')'           Punctuation
'\n'          Other

' '           Other
' '           Other
'Hello,'      Other
' '           Other
'${'          Comment.Preproc
'name'        Name
'}'           Comment.Preproc
'!'           Other
'\n'          Other

'#'           Comment.Preproc
'end'         Keyword
'\n'          Other

'${'          Comment.Preproc
'greeting'    Name
'('           Punctuation
"'"           Literal.String.Single
'world'       Literal.String.Single
"'"           Literal.String.Single
')'           Punctuation
'}'           Comment.Preproc
'\n'          Other

'\n'          Text

'#'           Comment.Preproc
'with'        Keyword
' '           Text
'y'           Name
'='           Operator
'7'           Literal.Number.Integer
';'           Punctuation
' '           Text
'z'           Name
'='           Operator
'x'           Name
'+'           Operator
'10'          Literal.Number.Integer
'\n'          Other

' '           Other
' '           Other
'$x'          Name.Variable
' '           Other
'$y'          Name.Variable
' '           Other
'$z'          Name.Variable
'\n'          Other

'#'           Comment.Preproc
'end'         Keyword
'\n'          Other