[MarkSweep]

I don't think .NET Framework is a supper set of all other implementions of .NET. For example, Mono's SIMD types are not in .NET Framework. It is even possible for types to exist in .NET Core that don't exist in .NET Framework.

I think you could say the following:

    .NET ECMA Spec ⊆ .NET Standard
    .NET Standard ⊆ .NET Core
    .NET Standard ⊆ Mono
    .NET Standard ⊆ Xamarin
    .NET Standard ⊆ .NET Framework

[smitherfield]

You're confusing intersection and union.

[MarkSweep]

Oh, derp, you're right.