Returns the smallest (closest to negative infinity) number with no
fractional part that is not less than the value of $arg. If type of $arg
is one of the four numeric types xs:float, xs:double, xs:decimal or
xs:integer the type of the result is the same as the type of $arg. If the
type of $arg is a type derived from one of the numeric types, the result
is an instance of the base numeric type.
For xs:float and xs:double arguments, if the argument is positive zero,
then positive zero is returned. If the argument is negative zero, then
negative zero is returned. If the argument is less than zero and greater
than -1, negative zero is returned.