Bug due to the fact that a variable can be "defined" and "read"
(in scheduling sense) by the same equation, without being a memory:
e.g., a clock defined as the result of a node application, together
with another result on this same clock.
Bug correction: basically removed the "assert false" on killed_vars,
decr_uses; do not count as "use" the self reads.
90dda27a3a