WHILE {
	Program = Cmd | BExp | AExp

	Cmd	(commands / statements)
		= Cmd ";" Cmd							-- seq
		| "if" BExp "then" Cmd "else" Cmd "end"	-- if
		| "while" BExp "do" Cmd "end"			-- while
		| var ":=" AExp							-- assign
		| "skip"								-- skip

	BExp
		= BExp "&&" BExp	-- and
		| BExp "||" BExp	-- or
        | "!" PriBExp		-- not
        | PriBExp

	PriBExp
		= "(" BExp ")"		-- paren
		| "&&" PriBExp		-- and
		| "||" PriBExp		-- or
		| AExp "=" AExp		-- eq
		| AExp ">" AExp		-- gt
		| AExp "<" AExp		-- lt
		| AExp ">=" AExp	-- geq
		| AExp "<=" AExp	-- leq
		| bool

	AExp (arithmetic expressions)
		= AExp "+" AExp		-- add
		| AExp "-" AExp		-- sub
		| AExp "*" PriAExp	-- mul
		| PriAExp

	PriAExp
		= "(" AExp ")"		-- paren
		| "+" PriAExp		-- pos
		| var
		| number

	bool (a bool value)
		= "true" ~var
		| "false" ~var

	var  (a variable)
		= letter+

	number	(positive and negative integers)
		= "-" digit+	--negative
		| digit+		--positive
}

    
x := 6;
y := 7;
z := 0;
while x > 0 do
  x := x - 1;
  v := y;
  while v > 0 do
    v := v - 1;
    z := z + 1
  end
end

    
    
while x > 0 do
  x := x - 1;
  v := y;
  while v > 0 do
    v := v - 1;
    z := z + 1
  end
end

    
    
x := 0;
y := 1;
z := 0;
while x < 100 do
  z := x + y;
  x := y;
  y := z
end

    
z := (x+3) * (y-2)

    
    
(2 - -4) * (11 - y)

    
    
!1=1 || (true && 3 <= 4)