theorem Th62:
  for A, t, u st A, GRZ-rules |- t & A, GRZ-rules |- t => u holds
      A, GRZ-rules |- u
proof
  let A, t, u;
  assume that
    A1: A, GRZ-rules |- t and
    A2: A, GRZ-rules |- t => u;
  A, GRZ-rules |- t '&' u by A1, A2, Th61;
  hence thesis by Th60;
end;
