theorem Th59:
  c in (succ b)\a iff a c= c & c c= b
  proof
    a c= c iff c nin a by ORDINAL6:4; then
    c in (succ b)\a iff c in succ b & a c= c by XBOOLE_0:def 5;
    hence thesis by ORDINAL1:22;
  end;
