Usage:
<representatives a b result
computes a minimal set of representatives for the
image of a modulo the image of b,IF THE FIRST CONTAINS
THE SECOND.
That is, if a and b are regarded as maps to
a free module F, then result is a map to
F whose columns correspond to a minimal set
of generators of (im a)/(im b).
Parameters:
a, b matrices with the same target (row space).
Output values:
result = matrix with the same target as a,b.
This script assumes that im a contains im b.
If one does not have this assumption, use
the script representatives instead.