projections redex (proj (mk_foo ...)) where mk_foo

projections redex (proj (mk_foo ...)) where mk_foo
is explicit (not the result of a reduction) are
always fired.

on the contrary, when the delta avoids the reduction
of a projetion (since the arg reduces to mk_foo but
for a smaller delta) we store in stack not the reduct
but its original version (before reducing it to delta=0).

This seems nice anyway, and a FIXME is left in the code