code: function symbolAlias(aliasFor) {
    return function(lexer, text) {
      var token = lexer.next_token(text);
      return sl.atom(aliasFor, {token: token});
    }
  }
docs: A symbol that's aliased to another.
  e.g. We allow them do assignment like either "(set var value)" or
  "(= var value)" by treating "=" as an alias for "set".
  To allow that the symbol table has:
  exports["="] = reader.symbolAlias("set").