val3(plus(X, Y), Context, Value) :-
  val3(X, Context, XValue),
  val3(Y, Context, YValue),
  Value is XValue + YValue.

val3(times(X, Y), Context, Value) :-
  val3(X, Context, XValue),
  val3(Y, Context, YValue),
  Value is XValue * YValue.

val3(const(X), _, X).

val3(var(X), Context, Value) :-
  lookup(X, Context, Value).

val3(let(X, Exp1, Exp2), Context, Value2) :-
  val3(Exp1, Context, Value1),
  val3(Exp2, [bind(X, Value1) | Context], Value2).

val3(fn(Formal, Body), _, fval(Formal, Body)).

val3(apply(Function, Actual), Context, Value) :-
  val3(Function, Context, fval(Formal, Body)),
  val3(Actual, Context, ParamValue),
  val3(Body, [bind(Formal, ParamValue)|Context], Value).

lookup(Variable, [bind(Variable, Value)|_], Value).
lookup(VarX, [bind(VarY, _)|Rest], Value) :-
  VarX \= VarY, lookup(VarX, Rest, Value).