With V1.1.31, fp64_pow(x,y) returns results if both x and y are negative, if y is an integer, e.g. fp64_pow(-2,-2) = 0.25, or fp64_pow(-2,-3) = -0.125. If y is not an integer, the result is NaN, e.g. fp64_pow(-2,-2.5) = NaN.
NB.: When y is not an integer, the correct mathematical result can no longer be represented as a real number but would be a complex number, e.g. (-2)^(-2.5) = 0 – 0.176776695297i.