as suggested by Yann in merge request !9.
When a parameterized symbol does not use its parameter,
fewer graph edges are created.
Add two tests that were rejected but can be accepted
(expansion_ok, expansion_unused)
and another test which was accepted already
(self_ground).
The effect of this change is marginal: in practice,
very few graphs are affected by this change.
Nevertheless, with this change, the safe-expansion check
might be correct and complete (who knows?).