Try all sort of things on it. Use alphabets in place of numbers, include a third number etc. And see for yourself what you get. That's the true hacker's spirit. Now create the $add() Identifier in following two different ways and see what you get: 

alias add return $calc($1 + $2 + $3 + $4) 


alias add { 
  if ($3) { 
    return This Identifier can add only two numbers. Cut the $3- off! 
    halt 
  } 
  else return $calc($1 + $2) 
}