theorem Th11:
  o is idempotent implies (o,D).:A is idempotent
proof
  assume
A1: o is idempotent;
  let f be Element of Funcs(A,D);
  thus ((o,D).:A).(f,f) = o.:(f,f) by Def2
    .= f by A1,Th6;
end;
