LPEG: fold capture

Deal all,

as far as I understand the documentation on fold captures in LPEG (http://www.inf.puc-rio.br/~roberto/lpeg/#cap-f), the folding function should be called if there is at least one capture in the pattern passed to lpeg.Cf ().

However, the real minimum seems to be two.

The following code

tostring (lpeg.P{'sum'
	 	, sum	= lpeg.Cf ( lpeg.V'operand' * lpeg.Cg ( lpeg.V'operator' * lpeg.V'operand' ) ^ 0, function (arg1, op, arg2)
	 		if op == '+' then
	 			return arg1 + arg2
	 		elseif op == '-' then
	 			return arg1 - arg2
	 		elseif op == nil then
				-- Special case: there is only one number in the _expression_:
				return 0
	 		end
		end )
	 , operand	= lpeg.C ( lpeg.R'09' ^ 1 / tonumber)
	 , operator = lpeg.C (lpeg.S'+-')
	}:match '4')
returns '4' (the whole line) rather than '0' whivh it is supposed to return if the string consists of only one number without arithmetc signs, which means that the folding function was never called.
Alexander Mashin