Bugfix Runtime error #1.

The following rule was not handled well:

A(x),B(x)->A(x~p!1),B(x!1)

because the site x in A in the lhs has no internal state, whereas it has one in the rhs.

The fix does not assume anymore that the same sites document their internal states.
It is done by using an asymetric fold2_fold instead of the symmetric one (fold2z_fold).