reserve V,X,Y for RealLinearSpace;
reserve u,u1,u2,v,v1,v2 for VECTOR of V;
reserve a for Real;
reserve V1,V2,V3 for Subset of V;
reserve x for object;
reserve W,W1,W2 for Subspace of V;
reserve w,w1,w2 for VECTOR of W;
reserve D for non empty set;
reserve d1 for Element of D;
reserve A for BinOp of D;
reserve M for Function of [:REAL,D:],D;
reserve B,C for Coset of W;

theorem Th61:
  u + v in v + W iff u in W
proof
  thus u + v in v + W implies u in W
  proof
    assume u + v in v + W;
    then ex v1 st u + v = v + v1 & v1 in W;
    hence thesis by RLVECT_1:8;
  end;
  assume u in W;
  hence thesis;
end;
