ext

Usage:
    <ext n a b c
A presentation of the module
Ext^n(a,b) is put into the
variable c.
Parameters:
            n = non-negative integer
            a, b = modules
Output values:
            c = module
A presentation of the module
Ext^n(A,B) is put into the
variable c, where A and B are presented by a and b.
We compute Ext as the homology
of the complex Hom(F*A,B), where F*A is a resolution of A.
If n=0, then Hom is used instead.