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+Require Import Coq.NArith.BinNat.
+Require Import Coq.Strings.Ascii.
+Require Import Coq.Strings.String.
+From stdpp Require Import fin_sets gmap option.
+From mininix Require Import expr maptools.
+
+Open Scope string_scope.
+Open Scope N_scope.
+Open Scope Z_scope.
+Open Scope nat_scope.
+
+Definition nonrecs : gmap string b_rhs → gmap string expr :=
+  omap (λ b, match b with B_Nonrec e => Some e | _ => None end).
+
+Lemma nonrecs_nonrec_fmap bs : nonrecs (B_Nonrec <$> bs) = bs.
+Proof.
+  induction bs using map_ind.
+  - done.
+  - unfold nonrecs.
+    rewrite fmap_insert.
+    rewrite omap_insert_Some with (y := x); try done.
+    by f_equal.
+Qed.
+
+Lemma rec_subst_nonrec_fmap bs : rec_subst (B_Nonrec <$> bs) = bs.
+Proof.
+  unfold rec_subst.
+  rewrite <-map_fmap_compose.
+  unfold compose.
+  by rewrite map_fmap_id.
+Qed.
+
+Definition ascii_ltb (c1 c2 : ascii) : bool :=
+  bool_decide (N_of_ascii c1 < N_of_ascii c2)%N.
+
+Fixpoint string_lt (s1 s2 : string) : bool :=
+  match s1, s2 with
+  | _, EmptyString => false
+  | EmptyString, String _ _ => true
+  | String c1 s1', String c2 s2' => ascii_ltb c1 c2 || string_lt s1' s2'
+  end.
+
+Fixpoint expr_eq (e1 e2 : expr) : bool :=
+  match e1, e2 with
+  | V_Null, V_Null => false
+  | V_Bool b1, V_Bool b2 => bool_decide (b1 = b2)
+  | V_Int n1, V_Int n2 => bool_decide (n1 = n2)
+  | V_Str s1, V_Str s2 => bool_decide (s1 = s2)
+  | V_Attrset bs1, V_Attrset bs2 => bool_decide (dom bs1 = dom bs2) &&
+      map_fold (λ x e1' acc, match bs2 !! x with
+                             | None => false
+                             | Some e2' => acc && expr_eq e1' e2'
+                             end) true bs1
+  | _, _ => false
+  end.
+
+Definition prelude : gmap string expr :=
+  {[ "true"  := X_V true;
+     "false" := X_V false;
+     "null"  := X_V V_Null;
+     "builtins" := X_Attrset
+       {[ "true"  := B_Nonrec true;
+          "false" := B_Nonrec false;
+          "null"  := B_Nonrec V_Null
+       ]}
+  ]}.