val1(plus(X,Y),Value) :-
  val1(X,XValue),
  val1(Y,YValue),
  Value is XValue + YValue.

val1(times(X,Y),Value) :-
  val1(X,XValue),
  val1(Y,YValue),
  Value is XValue * YValue.

val1(const(X),X).

/* val1(plus(plus(const(3), const(4)), times(const(2), const(5))), V). */