Added Krzysztof's theorems singleton_eq_iff, fst_type, snd_type Renamed doubleton_iff to doubleton_eq_iff
Added Krzysztof's theorems singleton_eq_iff, fst_type, snd_type
Renamed doubleton_iff to doubleton_eq_iff

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle