theorem
  (D1\/D2 is isotone & D1\/D2\/D3 is isotone &
  x is (m,SQ1,D1)-derivable & y is (m,SQ2,D2)-derivable &
  z is (n,{x,y},D3)-derivable) implies
  z is (m+n,SQ1\/SQ2,D1\/D2\/D3)-derivable by Lm46; ::#generalizing Lm28
