program ::= import stmts | simple stmts | compound stmts
import stmts ::= "import" module-name
simple stmts ::= assignment | expression
assignment ::= identifier "=" expression | identifier
compound stmts ::= if stmts | elif stmts | else stmts | function def | class def | function calls
if stmts ::= "if" expression | function call "{ ... }"
else stmts ::= "elif" experession | function call "{ ... }"
else ::= "else"