struct exp *e1, *e2, *e3;
  struct explist *l;

  e1 = mklet("y", mkconstant(10), mkvar("y"));

  l = NULL;
  l = cons(mkvar("y"), l);
  l = cons(e1, l);
  l = cons(mkvar("y"), l);
  e2 = mkopapp(isplus, l);

  e3 = mklet("y", mkconstant(2), e2);
  printf("%d\n", evalexp(e3) ); //should print 14

  return 0;