Slight improvement to print_equations()
Fix incorrect use of sparse() command. Fixes #1.
I had apparently assumed that sparse(n) would generate
a sparse n x n matrix (similar to zeros(n) or ones(n)).
Instead it creates a sparse 1x1 matrix containing only
the value n!  This lead to a spurious element in the
matrix, that, if not clobbered by another nonzero value
that might later be written to that position, would lead
to incorrect results.  Thanks to Chris Wipf for the test
case that uncovered this.