Attempt to replace one provider with another. This is typically used
        when resolving dependencies from multiple sources, e.g. A requires
        (B >= 1.0) while C requires (B >= 1.1).

        For successful replacement, ``provider`` must meet all the requirements
        which ``other`` fulfills.

        :param provider: The provider we are trying to replace with.
        :param other: The provider we're trying to replace.
        :param problems: If False is returned, this will contain what
                         problems prevented replacement. This is currently
                         a tuple of the literal string 'cantreplace',
                         ``provider``, ``other``  and the set of requirements
                         that ``provider`` couldn't fulfill.
        :return: True if we can replace ``other`` with ``provider``, else
                 False.

def try_to_replace(self, provider, other, problems):
        """
        Attempt to replace one provider with another. This is typically used
        when resolving dependencies from multiple sources, e.g. A requires
        (B >= 1.0) while C requires (B >= 1.1).

        For successful replacement, ``provider`` must meet all the requirements
        which ``other`` fulfills.

        :param provider: The provider we are trying to replace with.
        :param other: The provider we're trying to replace.
        :param problems: If False is returned, this will contain what
                         problems prevented replacement. This is currently
                         a tuple of the literal string 'cantreplace',
                         ``provider``, ``other``  and the set of requirements
                         that ``provider`` couldn't fulfill.
        :return: True if we can replace ``other`` with ``provider``, else
                 False.
        """
        rlist = self.reqts[other]
        unmatched = set()
        for s in rlist:
            matcher = self.get_matcher(s)
            if not matcher.match(provider.version):
                unmatched.add(s)
        if unmatched:
            # can't replace other with provider
            problems.add(('cantreplace', provider, other,
                          frozenset(unmatched)))
            result = False
        else:
            # can replace other with provider
            self.remove_distribution(other)
            del self.reqts[other]
            for s in rlist:
                self.reqts.setdefault(provider, set()).add(s)
            self.add_distribution(provider)
            result = True
        return result