Add helper/docs for permuting array with a Permutation?

[0joshuaolson1]

A more efficient atop(ptoa(p), wantPermuted) would be

function(p, wantPermuted){return p.map(function(i){return wantPermuted[i];});}

This makes a new array instead of permuting wantPermuted, but if Factoradic used an indexof reverse index here and a user wanted (to pass in) a wantPermuted copy anyway, that'd be two new arrays...

...I'll just write the indexof approach here sometime.