DEFAULT_COMPARATOR = function(a, b) {
  if (String(a) < String(b)) {
    return -1;
  } else if (String(a) > String(b)) {
    return 1;
  }
  return 0;
}
String comparison function used to compare values in the tree. This function
is used by default if no comparator is specified in the tree's constructor.

@param {T} a The first value.
@param {T} b The second value.
@return {number} -1 if a < b, 1 if a > b, 0 if a = b.
@template T
@const