Remark shift_symbol_address:
  forall id ofs delta,
  Genv.symbol_address ge id (Ptrofs.add ofs (Ptrofs.of_int delta)) = Val.add (Genv.symbol_address ge id ofs) (Vint delta).

Theorem eval_addsymbol: forall s ofs, unary_constructor_sound (addsymbol s ofs) (Val.add (Genv.symbol_address ge s ofs)).