Fake: What is the difference between fileset selectors: !!, ++ and !+ -

there different fileset selectors in fake. difference between these: !!, ++, !+?

i found these examples:

example 1:

// filesets let appreferences  =    !! "src/app/**/*.csproj"      ++ "src/app/**/*.fsproj"

example 2:

// files let appreferences =   !+ "./src/**/*.fsproj"     |> scan

example 3:

let filestozip =   !+ (builddir + "/**/*.*")     -- "*.zip"     |> scan

!+ "./src/**/*.fsproj"   |> scan

is obsolete , replaced !!

-- "*.zip"

excludes files fileset above.

Braziel