Compares numbers a and b and returns T if a = b. Otherwise, %null% is returned.

Often, floating-point calculations will introduce rounding errors. The delta
parameter provides a means of how close the numbers' values must be so that
they are considered equal. If delta is specified, the comparison will be
|b - a| < delta. If an argument is not a valid number, it will be treated as if
it were 0.