stringifies version
        :return: string of version

def to_string(self):
        """
        stringifies version
        :return: string of version
        """
        if self.major == -1:
            major_str = 'x'
        else:
            major_str = self.major
        if self.minor == -1:
            minor_str = 'x'
        else:
            minor_str = self.minor
        if self.patch == -1:
            patch_str = 'x'
        else:
            patch_str = self.patch
        return '{0}_{1}_{2}'.format(major_str, minor_str, patch_str)