class Sexp

def sub(pattern, repl)
  return repl.dup if pattern == self
  done = false
  new = self.map do |subset|
    if done then
      subset
    else
      case subset
      when Sexp then
        if pattern == subset then
          done = true
          repl.dup
        elsif pattern === subset then
          done = true
          subset.sub pattern, repl
        else
          subset
        end
      else
        subset
      end
    end
  end
  return Sexp.from_array(new)
end