lambda :: Parser Expr
lambda = do
  reservedOp "\\"
  x <- identifier
  reservedOp "->"
  e <- expr
  return (Lam x e)
lambda :: Parser Expr
lambda = do
  reservedOp "\\"
  x <- identifier
  reservedOp "->"
  e <- expr
  return (Lam x e)
lambda = do
  reservedOp "\\"
  x <- identifier
  reservedOp "->"
  e <- expr
  return (Lam x e)