reserve x,y,y1,y2 for object;
reserve GF for add-associative right_zeroed right_complementable Abelian
  associative well-unital distributive non empty doubleLoopStr,
  V,X,Y for Abelian add-associative right_zeroed right_complementable
  vector-distributive scalar-distributive scalar-associative scalar-unital non
  empty ModuleStr over GF;
reserve a for Element of GF;
reserve u,u1,u2,v,v1,v2 for Element of V;
reserve W,W1,W2 for Subspace of V;
reserve V1 for Subset of V;
reserve w,w1,w2 for Element of W;
reserve B,C for Coset of W;

theorem
  u in v + W iff ex v1 st v1 in W & u = v - v1
proof
  thus u in v + W implies ex v1 st v1 in W & u = v - v1
  proof
    assume u in v + W;
    then consider v1 such that
A1: u = v + v1 and
A2: v1 in W;
    take x = - v1;
    thus x in W by A2,Th22;
    thus thesis by A1,RLVECT_1:17;
  end;
  given v1 such that
A3: v1 in W and
A4: u = v - v1;
  - v1 in W by A3,Th22;
  hence thesis by A4;
end;
