Ensure that unary ops aux vars are never created when there is a lead

The transformation would be incorrect because of the expectation operator.

There was already a safety check, but it was not entirely correct. For example,
if “exp(y)” was appearing before “exp(y(+1))”, the check would not catch the
problem, because it happened after the substitution table had been filled. So
we now do the check before filling that table.