doc-src/Tutorial/Ifexpr/normal

removed weaker variant of subset_insert_iff

consts normal :: ifex => bool
primrec
"normal(CIF b) = True"
"normal(VIF x) = True"
"normal(IF b t e) = (normal t & normal e &
      (case b of CIF b => True | VIF x => True | IF x y z => False))"