Fixes propagation of zero into known non-zero variable
Fixes #7 (closed)
It could append that we solve 1=xy
by x=y^-1
because we know that y\not=0
but we later solve y=0
by the subtitution y\mapsto 0
. There is of course a contradiction but the order of the propagation can makes that it is the application of the substitution that finds it.