reserve x,y,y1,y2 for object;
reserve R for Ring;
reserve a for Scalar of R;
reserve V,X,Y for RightMod of R;
reserve u,u1,u2,v,v1,v2 for Vector of V;
reserve V1,V2,V3 for Subset of V;
reserve W,W1,W2 for Submodule of V;
reserve w,w1,w2 for Vector of W;

theorem Th20:
  u in W & v in W implies u + v in W
proof
  reconsider VW = the carrier of W as Subset of V by Def2;
  assume u in W & v in W;
  then
A1: u in the carrier of W & v in the carrier of W;
  VW is linearly-closed by Lm1;
  then u + v in the carrier of W by A1;
  hence thesis;
end;
