strict digraph sbox4 {
  x0 [shape=box, label="\N"];
  x1 [shape=box, label="\N"];
  x2 [shape=box, label="\N"];
  x3 [shape=box, label="\N"];

  t02 [label="\N\ny2\n1"];
  t01 [label="\N\ny3\n2"];

  t03 [label="\N\ny2\n3"];
  t06 [label="\N\ny3\n4"];

  t04 [label="\N\ny0\n5"];
  t05 [label="\N\nx3\n6"];
  t15 [label="\N\nx0\n7"];

  t11 [label="\N\nx1\n8"];

  t10 [label="\N\nx2\n9"];

  y3 [shape=box, label="\N\n10"];

  t13 [label="\N\ny2\n11"];

  t08 [label="\N\ny1\n12"];

  y2 [shape=box, label="\N\n13"];

  t12 [label="\N\ny1\n14"];
  t16 [label="\N\ny1\n15"];
  y1 [shape=box, label="\N\n16"];

  t09 [label="\N\ny0\n17"];
  t14 [label="\N\ny0\n18"];
  y0 [shape=box, label="\N\n19"];
  
  x0  -> t01; x1  -> t01;  // t01 = x0  | x1; 
  x1  -> t02; x2  -> t02;  // t02 = x1  | x2; 
  x0  -> t03; t02 -> t03;  // t03 = x0  ^ t02;
  x1  -> t04; x3  -> t04;  // t04 = x1  ^ x3; 
  x3  -> t05; t03 -> t05;  // t05 = x3  | t03;
  x3  -> t06; t01 -> t06;  // t06 = x3  & t01;
  t03 -> y3;  t06 -> y3;   // y3  = t03 ^ t06;
  y3  -> t08; t04 -> t08;  // t08 = y3  & t04;
  t04 -> t09; t05 -> t09;  // t09 = t04 & t05;
  x2  -> t10; t06 -> t10;  // t10 = x2  ^ t06;
  x1  -> t11; x2  -> t11;  // t11 = x1  & x2; 
  t04 -> t12; t08 -> t12;  // t12 = t04 ^ t08;
  t11 -> t13; t03 -> t13;  // t13 = t11 | t03;
  t10 -> t14; t09 -> t14;  // t14 = t10 ^ t09;
  x0  -> t15; t05 -> t15;  // t15 = x0  & t05;
  t11 -> t16; t12 -> t16;  // t16 = t11 | t12;
  t13 -> y2;  t08 -> y2;   // y2  = t13 ^ t08;
  t15 -> y1;  t16 -> y1;   // y1  = t15 ^ t16;
  t14 -> y0;               // y0  =     ~ t14;
}