rn_expr :: Parser (AST' Text)
return_expr = do
    reserved "return" <|> reserved "dirac"
    Dirac <$> expr

te
return_expr :: Parser (AST' Text)
return_expr = do
    reserved "return" <|> reserved "dirac"
    Dirac <$> expr
return_expr = do
    reserved "return" <|> reserved "dirac"
    Dirac <$> expr