code: function substr(val, start, length){
    utils.assertString(val, 'val');
    utils.assertType(start, 'unit', 'start');
    length = length && length.val;
    var res = val.string.substr(start.val, length);
    return val instanceof nodes.Ident
        ? new nodes.Ident(res)
        : new nodes.String(res);
  }
docs: Returns substring of the given `val`.
  
  @param {String|Ident} val
  @param {Number} start
  @param {Number} [length]
  @return {String|Ident}
  @api public