ROOTS

no need for subset operation as a primitive in Nitpick, esp. that its implementation was unsound (cf. Rene Thiemann's 16 Oct 2014 email on isabelle mailing list)

src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools