1) Strings - summary

   set Sx, Sy        # alias Sx to Sy
   set Sx, "str"     # set Sx to COW copy of "str"
   set Px, Sy        # CHANGED make Px refer to Sy
   assign Px, Sy     # set Px to COW copy of Sy

2) PMC summary

   set Px, Py        # alias Px to Py
   assign Px, Py     # set Px to value of Py  := Px->vtable->assign_pmc
   setref Px, Py     # make Px refer to Py    := Px->vtable->set_pmc

The latter is the opposite of deref, which now calls the get_pmc()
vtable. C<assign> for reference PMCs is now just passed on.

leo