import_stmt ::= "import" module ["as" identifier] ("," module ["as" identifier])*
| "from" relative_module "import" identifier ["as" identifier]
("," identifier ["as" identifier])*
| "from" relative_module "import" "(" identifier ["as" identifier]
("," identifier ["as" identifier])* [","] ")"
| "from" module "import" "*"
module ::= (identifier ".")* identifier
relative_module ::= "."* module | "."+