function almostEqual(a, b) {
  return Math.abs(a - b) <= 10;
}
Sometimes a child will reach out of the parent by a few px, but still
clearly belong to the same tap area in the users's eyes.
We can be quite generous here, since merging too much tends to cause false
passes instead of false failures (because there are more fingers)
@param {number} a
@param {number} b