Power predicate

[mlliarm]

Implement the power(X, N) predicate, where X is an interval and N is a number, calculating thus X^N.

The definition of this can be found in Shary2021 page 40, and it is pretty much defined as the N-times multiplication of X with itself:

X^N = X*X*...*X, N times.

This should be calculated recursively:

X^N = X*X^(N-1).