It is located in Powerpack F #, which is used by the F # team for "intermittent" code that will be released and updated more often than the main release of F # and libraries. (Note that the code is still high-quality code, you should ignore most connotations, which in this case are "unstable"). This is just a way for the F # team to get more experimental code faster as soon as F # is actually released.
Powerpack codeplex