theorem Th27:
  for a st f.a [= a for O1, O2 st O1 c< O2 & not (f, O2)-.a
  is_a_fixpoint_of f holds (f, O1)-.a <> (f, O2)-.a
proof
  let a;
  assume
A1: f.a [= a;
  let O1, O2;
A2: (f, succ O1)-.a [= (f, O1)-.a by A1,Th25,XBOOLE_1:7;
  assume that
A3: O1 c< O2 and
A4: not (f, O2)-.a is_a_fixpoint_of f and
A5: (f, O1)-.a = (f, O2)-.a;
  O1 in O2 by A3,ORDINAL1:11;
  then succ O1 c= O2 by ORDINAL1:21;
  then (f, O2)-.a [= (f, succ O1)-.a by A1,Th25;
  then (f, O1)-.a = (f, succ O1)-.a by A5,A2,LATTICES:8;
  then (f, O1)-.a = f.(f, O1)-.a by Th16;
  hence contradiction by A4,A5;
end;
