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

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),Context,fval(Formal,Body,Context)).

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