reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;

theorem Th6:
  for a,b being natural Number holds Seg a = Seg b implies a = b
proof
  let a,b be natural Number;
  assume Seg a = Seg b;
  then a <= b & b <= a by Th5;
  hence thesis by XXREAL_0:1;
end;
