rewrites: simplify collapsing of diistributors

We can use the two permutation rules to 'empty' the former distributors before
collapsing them, so no need to be so general (and hence no need for the
sidecondotion).
rewrite: restrict the second permutation rule

I noticed that the side-condition I removed in 02193f is needed in the second
permutation rule. Moreover, rather than forbidding \x_i, we should be slightly
more general and forbid x_i, as it might occur in another diistributor.