Returns the arithmetic difference of its operands: (
$arg1 - $arg2).
General rules: see .
xs:double values, if one of the operands is a
zero or a finite number and the other is
infinity of the appropriate sign is returned. If both operands are
NaN is returned. If one of the operands is
INF and the other is
-INF, an infinity of the appropriate
sign is returned.