theorem
  for A, t, u st A, GRZ-rules |- t '&' u holds A, GRZ-rules |- u '&' t
proof
  let A, t, u;
  assume A, GRZ-rules |- t '&' u;
  then A, GRZ-rules |- t & A, GRZ-rules |- u by Th60;
  hence thesis by Th60;
end;
