theorem Th19:
  [p '&' q,Sub] = CQCSub_&([p,Sub],[q,Sub])
proof
  set S1 = [p,Sub];
  set S2 = [q,Sub];
A1: S1`1 = p & S2`1 = q;
A2: S1`2 = Sub & S2`2 = Sub;
  then CQCSub_&(S1,S2) = Sub_&(S1,S2) by SUBLEMMA:def 3;
  hence thesis by A2,A1,SUBSTUT1:def 21;
end;
