extern :: Parser Expr
extern = do
  reserved "extern"
  name <- identifier
  args <- parens $ many identifier
  return $ Extern name args
extern :: Parser Expr
extern = do
  reserved "extern"
  name <- identifier
  args <- parens $ many identifier
  return $ Extern name args
extern = do
  reserved "extern"
  name <- identifier
  args <- parens $ many identifier
  return $ Extern name args