Syntax: FFT n pexpr1 pexpr2 WORD1 WORD2

   Fourier transform 2 vectors (treated as the real and imaginary parts
of a complex vector), returning the answer in the two vectors WORD1 and
WORD2. The input vectors may be the names of vectors or expressions in
parentheses. The direction is specified by n, either +1 for a forward
transform, or -1 for an inverse.

   The dimension of the vectors need not be a power of 2, but the transform
is more efficient if it is. The worst case is when n is prime, in which case
this command does a slow Fourier transform in O(n^2) time.