Similar to 4c49accf8997da21be19be0396b2a88f33b9f949 and
615cf4bdce0de86e57bdc27e008a35dd713e483f this avoids the use of
non-reserved names as function parameters. For this function the
parameter names do actually provide useful information, because
without the names it's not clear which parameter is the numerator and
which the denominator.  So rather than just remove the names, this
prefixes them with double-underscores.