[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
  (s) expr.isDouble = term.isDouble
  (i) term.type = if expr.isDouble then double else int

 Expr1 ::= Term [+-] Expr2
  (s) expr1.isDouble = term.isDouble or expr2.isDouble
  (i) term.type = expr1.isDouble then double else int
  (i) expr2.type = expr1.isDouble then double else int

 Factor ::= (Expr)
  (s) factor.isDouble = expr.isDouble
  (i) expr.type = factor.isDouble then double else int

 Factor ::= Value
  (s) factor.isDouble = value.isDouble
  (i) value.type = factor.isDouble then double else int

 Value :: Decimal Integer
  (s) value.isDouble = false
  (i) value.type = value.type == double then double else int

 Value :: Double
  (s) value.isDouble = true
  (i) value.type = value.type == double then double else int
}}}