theorem Th24:
  D = {p} implies <.D.) = <.p.)
proof
  assume
A1: D = {p};
  D c= <.p.)
  proof
    let x be object;
    assume x in D;
    then x = p by A1,TARSKI:def 1;
    hence thesis;
  end;
  hence <.D.) c= <.p.) by Def4;
  p in D by A1,TARSKI:def 1;
  hence thesis by Th23;
end;
