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

val2(plus(X,Y),Context,Value) :-
  val2(X,Context,XValue),
  val2(Y,Context,YValue),
  Value is XValue + YValue.

val2(times(X,Y),Context,Value) :-
  val2(X,Context,XValue),
  val2(Y,Context,YValue),
  Value is XValue * YValue.

val2(const(X),_,X).

val2(var(X),Context,Value) :-
  lookup(X,Context,Value).

val2(let(X,Exp1,Exp2),Context,Value2) :-
  val2(Exp1,Context,Value1),
  val2(Exp2,[bind(X,Value1)|Context],Value2).