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(*                            Syntactic heaps                             *)
(* Pointers and heap variables are syntactified as indices in the context *)
(* Disjoint union is syntactified as concatenation of lists               *)
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Definition synheap := seq elem.
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(* Validity of indices:                                                      *)
(*   valid i t  iif all the indices in t are in bounds of the contexts lists *)
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Fixpoint valid_ptrs i t :=
match t with
Pts sx _ :: s => (sx < size (ptr_ctx i)) && valid_ptrs i s
| Var _ :: s => valid_ptrs i s
| _ => true
end.
Fixpoint valid_heaps i t :=
match t with
Pts _ _ :: s => valid_heaps i s
| Var v :: s => (v < size (heap_ctx i)) && valid_heaps i s
| _ => true
end.
Definition valid i t := valid_ptrs i t && valid_heaps i t.

Lemma valid_subctx i j t : subctx i j -> valid i t -> valid j t.
Definition hlook := [fun i => onth (heap_ctx i)].