theorem Th7:
  for l1, l2 being Element of NAT holds locnum(l1,T) =
  locnum(l2,T) implies l1 = l2
proof
  let l1, l2 be Element of NAT;
  assume
A1: locnum(l1,T) = locnum(l2,T);
  il.(T,locnum(l1,T)) = l1 by Def5;
  hence thesis by A1,Def5;
end;
