Preprocessor simplification incorrectly treats foward-looking equations.

The file simple_impinf.mod contains the line

mu_x(+1) = 0;

Starting in 4.6, the preprocessor simplifies constant variables. The problem is that it interprets the above equation to imply mu_x=0 and drops the variable from the lead_lag_incidence. But something that is 0 in expectations is not necessarily 0 in all states of the world.