function feetLengths(contour, leftFeet, rightFeet) {
  return [
    leftFeet.map(index => {
      const pt2 = contour[index];
      const pt4 = contour[mod(index+2, contour.length)];
      return pt4.y - pt2.y;
    }),
    rightFeet.map(index => {
      const pt2 = contour[index];
      const pt4 = contour[mod(index-2, contour.length)];
      return pt4.y - pt2.y;
    })
  ];
}
Feet lengths
@param { Contour } contour 
@param { number[] } leftFeet 
@param { number[] } rightFeet 
@returns { [ number[], number[] ] }