Lunar言語の中間表現であり、ここからLLVM IRへ変換。
- IR := TOP*
 - TOP := GLOBAL | THREADLOCAL | IMPORT | TOPSTATEMENT | DEFUN | EXPR
 - TOPSTATEMENT := LET | COND | WHILE | SELECT | BLOCK | STRUCT | CUNION | UNION
 - STATEMENT := LET | COND | WHILE | BREAK | SELECT | RETURN | STRUCT | CUNION | UNION | BLOCK | LEAP