public void PackageConflicts(PackageIdentifier identifier, CallStack failingCallStack)
        {
            PackageResolutionStacks existingCompatible = CompatiblePackageVersions[identifier];

            _compatiblePackageVersions.Pop();
            CompatiblePackageVersions.Remove(identifier);

            _incompatiblePackages.Pop();
            IncompatiblePackageVersions.Add(identifier, existingCompatible.Successful, existingCompatible.Failed.Concat(failingCallStack));
        }
 static PackageResolutionStacks()
 {
     Null = new PackageResolutionStacks();
 }
 static PackageResolutionStacks()
 {
     Null = new PackageResolutionStacks();
 }