theorem Th15:
  Gen x,y implies ex v st Orte(x,y,v) = u
proof
  assume
A1: Gen x,y;
  take v= -Orte(x,y,u);
  thus Orte(x,y,v) = -Orte(x,y,Orte(x,y,u)) by A1,Th9
    .= -(-u) by A1,Th14
    .= u by RLVECT_1:17;
end;
