During the lecture, we make a quick edit of BWT.java to reduce the
memory usage (in transform) from quadratic to linear, so that we
could then compute the forward transform on mobydick.txt

The original quadratic-space version is BWT.java.orig

A second interpretation of array T, in the last paragraph:
if we stably sort the chars in L, then L[j] goes to F[T[j]].

VL suggested we try BWT on words instead of chars, that is a good
idea, except we need to define "words" so that they survive
rearrangement intact.  After that, we just need stable sorting.