Returns one of :less, :greater, :equal, or :unequal according as x
is less than, greater than, or equal to y, or none of these. While the
ordering does not have to be total, it must be consistent: for two values
A and B that compare :unequal to each other, for any third value C, if A
compares :less or :greater to C, then B must compare to C the same way;
and no more than one of A and B can compare :equal to C.