F# project won't build with 'Lazy`2' required and unavailable

Question

This is an F# test project which I'm trying to 'share' between VS2013 and VS2012. I'm not referencing Lazy anything in my code, so I'm not sure what's up.

I believe it's to do with having the latest version of xunit (2.0) referenced.

The type 'Lazy`2' is required here and is unavailable. You must add a reference 
to assembly 'System.ComponentModel.Composition, Version=4.0.0.0, Culture=neutral, 
PublicKeyToken=b77a5c561934e089'.   FSC 1   1   MyProj

also

A reference to the type 'System.Exception' in assembly 'System.Runtime' was found, 
but the type could not be found in that assembly
C:\MyProj\startup   1   1   MyProj

Answer 1

Never found a solution, I just abandoned VS2012 and recreated the project in VS2013 only.

Note that the project was x64, which may not have helped.