trim -- minimize generators and relations

There are two ways to present a module M over a ring. One way is to take a free module F (whose generators are called the generators) and form the quotient M = F/H by a submodule H of F (whose generators are called the relations). Another way is take a free module F, a submodule G of F (whose generators are called the relations), a submodule H of F (whose generators are called the relations), and form the subquotient module M = (G+H)/H, obtained also as the image of G in F/H. The purpose of trim is to minimize presentations of the latter type. This applies also to rings and ideals.