def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
def __init__(self, head):
        self.head = head
        self.lsize = 0
        while head.next:
            head = head.next
            self.lsize += 1

        self.m1_idx = None
        self.m2_idx = None
        if self.lsize > self._largesize:
            self.m1_idx = self.lsize / 3   # start from 1/3
            self.m1 = self._getN(self.m1_idx)
            self.m2_idx = self.m1_idx * 2  # start from 2/3
            self.m2 = self._getN(self.m2_idx)