reserve V for non empty RLSStruct;
reserve x,y,y1 for set;
reserve v for VECTOR of V;
reserve a,b for Real;

theorem Th21:
  for V being add-associative right_zeroed right_complementable
  non empty addLoopStr, v,w being Element of V holds v - w = 0.V implies v = w
proof
  let V be add-associative right_zeroed right_complementable non empty
  addLoopStr;
  let v,w be Element of V;
  assume v - w = 0.V;
  then - v = - w by Def10;
  hence thesis by Th18;
end;
