(* we don't handle cases where t1 doesn't evaluate to a Pair *)
let (Pair (index1, index2), s') = eval t1 g s
(* note that s' may be different from s, if t1 itself contained any mutation operations *)
(* we don't handle cases where t1 doesn't evaluate to a Pair *)
let (Pair (index1, index2), s') = eval t1 g s
(* note that s' may be different from s, if t1 itself contained any mutation operations *)