theorem
  W1 is Submodule of W3 implies W1 /\ W2 is Submodule of W3
proof
  A1: W1 /\ W2 is Submodule of W1 by ZMODUL01:105;
  assume W1 is Submodule of W3;
  hence thesis by A1,ZMODUL01:42;
end;
