Return both variable names used in the #for loop in the
        current line.

def _compile(cls, lines):
        '''Return both variable names used in the #for loop in the
        current line.'''
        m = cls.RE_FOR.match(lines.current)
        if m is None:
            raise DefineBlockError(
                'Incorrect block definition at line {}, {}\nShould be '
                'something like: #for @item in @items:'
                .format(lines.pos, lines.current))
        return m.group(1), m.group(2).replace('.', '-')