homology

Usage:
    <homology b c f g h
(brief description)
Parameters:
            b,c,f,g = modules
Output values:
            h = module
If b,c are maps presenting modules B,C,
and f,g are maps inducing f: A->B and g:B->C,
(for some module A)
then a presentation of the homology is computed
and assigned to h.