The evaluable function (^)/2.

Dear GNU Prolog Developer,

Accidentially, by some testing, I saw that GNU Prolog
produces the following results:

GNU Prolog 1.4.4 (64 bits)
Compiled Apr 23 2013, 16:05:07 with cl
?- X is 2^(-2).
X = 0
?- X is 2^(-1.5).
X = 0.35355339059327379

To my understanding supporting negative values and/or
non integer values for the second argument of (^)/2 is optional.

But returning zero in the first case above seems a little bit
odd to me. SWI Prolog produces the following results:

Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.3.10)
Copyright (c) 1990-2015 University of Amsterdam, VU Amsterdam
?- X is 2^(-2).
X = 0.25.
?- X is 2^(-1.5).
X = 0.3535533905932738.

Bye