reserve T for non empty Abelian
  add-associative right_zeroed right_complementable RLSStruct,
  X,Y,Z,B,C,B1,B2 for Subset of T,
  x,y,p for Point of T;

theorem Th19:
  x in X implies B+x c= B (+) X
proof
  assume
A1: x in X;
  let y be object;
  assume y in B+x;
  then ex y1 being Point of T st y = y1+x & y1 in B;
  hence thesis by A1;
end;
