diff --git a/Imp.v b/Imp.v
new file mode 100644
index 0000000..494c92d
--- /dev/null
+++ b/Imp.v
@@ -0,0 +1,218 @@
+Require Import Frap.
+
+Set Implicit Arguments.
+
+
+Inductive arith : Set :=
+| Const (n : nat)
+| Var (x : var)
+| Plus (e1 e2 : arith)
+| Minus (e1 e2 : arith)
+| Times (e1 e2 : arith).
+
+Inductive cmd :=
+| Skip
+| Assign (x : var) (e : arith)
+| Sequence (c1 c2 : cmd)
+| If (e : arith) (then_ else_ : cmd)
+| While (e : arith) (body : cmd).
+
+Coercion Const : nat >-> arith.
+Coercion Var : var >-> arith.
+Infix "+" := Plus : arith_scope.
+Infix "-" := Minus : arith_scope.
+Infix "*" := Times : arith_scope.
+Delimit Scope arith_scope with arith.
+Notation "x <- e" := (Assign x e%arith) (at level 75).
+Infix ";;" := Sequence (at level 76). (* This one changed slightly, to avoid parsing clashes. *)
+Notation "'when' e 'then' then_ 'else' else_ 'done'" := (If e%arith then_ else_) (at level 75, e at level 0).
+Notation "'while' e 'loop' body 'done'" := (While e%arith body) (at level 75).
+
+Definition valuation := fmap var nat.
+Fixpoint interp (e : arith) (v : valuation) : nat :=
+  match e with
+  | Const n => n
+  | Var x =>
+    match v $? x with
+    | None => 0
+    | Some n => n
+    end
+  | Plus e1 e2 => interp e1 v + interp e2 v
+  | Minus e1 e2 => interp e1 v - interp e2 v
+  | Times e1 e2 => interp e1 v * interp e2 v
+  end.
+
+Inductive eval : valuation -> cmd -> valuation -> Prop :=
+| EvalSkip : forall v,
+  eval v Skip v
+| EvalAssign : forall v x e,
+  eval v (Assign x e) (v $+ (x, interp e v))
+| EvalSeq : forall v c1 v1 c2 v2,
+  eval v c1 v1
+  -> eval v1 c2 v2
+  -> eval v (Sequence c1 c2) v2
+| EvalIfTrue : forall v e then_ else_ v',
+  interp e v <> 0
+  -> eval v then_ v'
+  -> eval v (If e then_ else_) v'
+| EvalIfFalse : forall v e then_ else_ v',
+  interp e v = 0
+  -> eval v else_ v'
+  -> eval v (If e then_ else_) v'
+| EvalWhileTrue : forall v e body v' v'',
+  interp e v <> 0
+  -> eval v body v'
+  -> eval v' (While e body) v''
+  -> eval v (While e body) v''
+| EvalWhileFalse : forall v e body,
+  interp e v = 0
+  -> eval v (While e body) v.
+
+Inductive step : valuation * cmd -> valuation * cmd -> Prop :=
+| StepAssign : forall v x e,
+  step (v, Assign x e) (v $+ (x, interp e v), Skip)
+| StepSeq1 : forall v c1 c2 v' c1',
+  step (v, c1) (v', c1')
+  -> step (v, Sequence c1 c2) (v', Sequence c1' c2)
+| StepSeq2 : forall v c2,
+  step (v, Sequence Skip c2) (v, c2)
+| StepIfTrue : forall v e then_ else_,
+  interp e v <> 0
+  -> step (v, If e then_ else_) (v, then_)
+| StepIfFalse : forall v e then_ else_,
+  interp e v = 0
+  -> step (v, If e then_ else_) (v, else_)
+| StepWhileTrue : forall v e body,
+  interp e v <> 0
+  -> step (v, While e body) (v, Sequence body (While e body))
+| StepWhileFalse : forall v e body,
+  interp e v = 0
+  -> step (v, While e body) (v, Skip).
+
+Hint Constructors trc step eval.
+
+Lemma step_star_Seq : forall v c1 c2 v' c1',
+  step^* (v, c1) (v', c1')
+  -> step^* (v, Sequence c1 c2) (v', Sequence c1' c2).
+Proof.
+  induct 1; eauto.
+  cases y; eauto.
+Qed.
+
+Hint Resolve step_star_Seq.
+
+Theorem big_small : forall v c v', eval v c v'
+  -> step^* (v, c) (v', Skip).
+Proof.
+  induct 1; eauto 6 using trc_trans.
+Qed.
+
+Lemma small_big'' : forall v c v' c', step (v, c) (v', c')
+                                      -> forall v'', eval v' c' v''
+                                                     -> eval v c v''.
+Proof.
+  induct 1; simplify;
+  repeat match goal with
+         | [ H : eval _ _ _ |- _ ] => invert1 H
+         end; eauto.
+Qed.
+
+Hint Resolve small_big''.
+
+Lemma small_big' : forall v c v' c', step^* (v, c) (v', c')
+                                     -> forall v'', eval v' c' v''
+                                                    -> eval v c v''.
+Proof.
+  induct 1; eauto.
+  cases y; eauto.
+Qed.
+
+Hint Resolve small_big'.
+
+Theorem small_big : forall v c v', step^* (v, c) (v', Skip)
+                                   -> eval v c v'.
+Proof.
+  eauto.
+Qed.
+
+Definition trsys_of (v : valuation) (c : cmd) : trsys (valuation * cmd) := {|
+  Initial := {(v, c)};
+  Step := step
+|}.
+
+Inductive context :=
+| Hole
+| CSeq (C : context) (c : cmd).
+
+Inductive plug : context -> cmd -> cmd -> Prop :=
+| PlugHole : forall c, plug Hole c c
+| PlugSeq : forall c C c' c2,
+  plug C c c'
+  -> plug (CSeq C c2) c (Sequence c' c2).
+
+Inductive step0 : valuation * cmd -> valuation * cmd -> Prop :=
+| Step0Assign : forall v x e,
+  step0 (v, Assign x e) (v $+ (x, interp e v), Skip)
+| Step0Seq : forall v c2,
+  step0 (v, Sequence Skip c2) (v, c2)
+| Step0IfTrue : forall v e then_ else_,
+  interp e v <> 0
+  -> step0 (v, If e then_ else_) (v, then_)
+| Step0IfFalse : forall v e then_ else_,
+  interp e v = 0
+  -> step0 (v, If e then_ else_) (v, else_)
+| Step0WhileTrue : forall v e body,
+  interp e v <> 0
+  -> step0 (v, While e body) (v, Sequence body (While e body))
+| Step0WhileFalse : forall v e body,
+  interp e v = 0
+  -> step0 (v, While e body) (v, Skip).
+
+Inductive cstep : valuation * cmd -> valuation * cmd -> Prop :=
+| CStep : forall C v c v' c' c1 c2,
+  plug C c c1
+  -> step0 (v, c) (v', c')
+  -> plug C c' c2
+  -> cstep (v, c1) (v', c2).
+
+Hint Constructors plug step0 cstep.
+
+Theorem step_cstep : forall v c v' c',
+  step (v, c) (v', c')
+  -> cstep (v, c) (v', c').
+Proof.
+  induct 1; repeat match goal with
+                   | [ H : cstep _ _ |- _ ] => invert H
+                   end; eauto.
+Qed.
+
+Hint Resolve step_cstep.
+
+Lemma step0_step : forall v c v' c',
+  step0 (v, c) (v', c')
+  -> step (v, c) (v', c').
+Proof.
+  invert 1; eauto.
+Qed.
+
+Hint Resolve step0_step.
+
+Lemma cstep_step' : forall C c0 c,
+  plug C c0 c
+  -> forall v' c'0 v c', step0 (v, c0) (v', c'0)
+  -> plug C c'0 c'
+  -> step (v, c) (v', c').
+Proof.
+  induct 1; simplify; repeat match goal with
+                             | [ H : plug _ _ _ |- _ ] => invert1 H
+                             end; eauto.
+Qed.
+
+Hint Resolve cstep_step'.
+
+Theorem cstep_step : forall v c v' c',
+  cstep (v, c) (v', c')
+  -> step (v, c) (v', c').
+Proof.
+  invert 1; eauto.
+Qed.
diff --git a/OperationalSemantics.v b/OperationalSemantics.v
index a34ea6d..d40ba3e 100644
--- a/OperationalSemantics.v
+++ b/OperationalSemantics.v
@@ -827,7 +827,7 @@ Lemma step0_step : forall v c v' c',
   step0 (v, c) (v', c')
   -> step (v, c) (v', c').
 Proof.
-  induct 1; constructor; assumption.
+  invert 1; constructor; assumption.
 Qed.
 
 Lemma cstep_step' : forall C c0 c,
@@ -853,7 +853,7 @@ Theorem cstep_step : forall v c v' c',
   cstep (v, c) (v', c')
   -> step (v, c) (v', c').
 Proof.
-  induct 1.
+  invert 1.
   eapply cstep_step'.
   eassumption.
   eassumption.
@@ -880,7 +880,7 @@ Lemma step0_step_snazzy : forall v c v' c',
   step0 (v, c) (v', c')
   -> step (v, c) (v', c').
 Proof.
-  induct 1; eauto.
+  invert 1; eauto.
 Qed.
 
 Hint Resolve step0_step_snazzy.
@@ -902,7 +902,7 @@ Theorem cstep_step_snazzy : forall v c v' c',
   cstep (v, c) (v', c')
   -> step (v, c) (v', c').
 Proof.
-  induct 1; eauto.
+  invert 1; eauto.
 Qed.
 
 
@@ -1166,7 +1166,7 @@ Module Concurrent.
     step0 (v, c) (v', c')
     -> step (v, c) (v', c').
   Proof.
-    induct 1; eauto.
+    invert 1; eauto.
   Qed.
 
   Hint Resolve step0_step.
@@ -1188,6 +1188,6 @@ Module Concurrent.
     cstep (v, c) (v', c')
     -> step (v, c) (v', c').
   Proof.
-    induct 1; eauto.
+    invert 1; eauto.
   Qed.
 End Concurrent.
diff --git a/_CoqProject b/_CoqProject
index 6e472d4..4391fe0 100644
--- a/_CoqProject
+++ b/_CoqProject
@@ -5,6 +5,7 @@ Sets.v
 Relations.v
 Invariant.v
 ModelCheck.v
+Imp.v
 Frap.v
 BasicSyntax_template.v
 BasicSyntax.v