function type(s, f) {
    var x = symbol(s, 0);
    x.type = s;
    x.nud = f;
    return x;
  }
Convenience function for defining "type" symbols--those that describe
literal values.

@param {string} s - the name of the symbol
@param {function} f - the first null denotation function for the symbol;
                      see the `expression` function for more detail

@returns {object} - the object describing the JSHint symbol (provided to
                    support cases where further refinement is necessary)