Sparse structural rank.
r = SPRANK(A) is the structural rank of the sparse matrix A.
Also known as maximum traversal, maximum assignment, and
size of a maximum matching in the bipartite graph of A.
Always sprank(A) >= rank(A), and in exact arithmetic
sprank(A) == rank(sprandn(A)) with probability one.
See also DMPERM2AMLGCO.