theorem Th22:
  Valid(p '&' p,J) = Valid(p,J)
proof
  now
    let v;
    thus Valid(p '&' p,J).v = (Valid(p,J).v) '&' (Valid(p,J).v) by Th12
      .= Valid(p,J).v;
  end;
  hence thesis by FUNCT_2:63;
end;
