text: function sum(term, a, next, b) {
      return a > b
             ? 0
             : term(a) + sum(term, next(a), next, b);
  }
  function inc(n) {
      return n + 1;
  }
  function identity(x) {
      return x;
  }
  function sum_integers(a, b) {
      return sum(identity, a, inc, b);
  }
  
  sum_integers(1, 10);