  Given free modules $M\subseteq L$ of finite rank $f\geq 1$ over a principal
ideal domain $R$, we give a procedure to construct a basis of $L$ from a basis
of $M$ assuming the invariant factors or elementary divisors of $L/M$ are
known. Given a matrix $A\in M_{m,n}(R)$ of rank $r$, its nullspace~$L$ in $R^n$
is a free $R$-module of rank~$f=n-r$. We construct a free submodule $M$ of $L$
of rank~$f$ naturally associated to $A$ and whose basis is easily computable,
we determine the invariant factors of the quotient module $L/M$, and then
indicate how to apply the previous procedure to build a basis of $L$ from one
of $M$.
