To locate the Registration class:

1. Check References: In Solution Explorer, look under "References" for Plural.CoreLib. If it’s there, the class is likely from that library.

2. NuGet Packages: Go to Tools > NuGet Package Manager and check if Plural.CoreLib is listed under installed packages.

3. Object Browser: Open the Object Browser (View > Object Browser) and search for Plural.CoreLib to confirm the class is defined there.