code: function math(n, fn){
    utils.assertType(n, 'unit', 'n');
    utils.assertString(fn, 'fn');
    return new nodes.Unit(Math[fn.string](n.val), n.type);
  }
docs: Apply Math `fn` to `n`.
  
  @param {Unit} n
  @param {String} fn
  @return {Unit}
  @api private