[http://www.aistudy.com/linguistics/chomsky_hierarchy.htm Chomsky Hierarchy 참고]

{{{
< EBNF >
 Expr ::= Term([+|-] Expr)?
 Term ::= Factor ([*|/] Term)?
 Factor :: = (Expr)
           | Value
 Value ::= Integer | Double

< >
 Expr  ::= Term
  expr.isDouble = term.isDouble

 Expr1 ::= Term [+-] Expr2
  expr1.isDouble = term.isDouble or expr2.isDouble

 
  
}}}