reserve n,m,k for Nat,
        p,q for n-element XFinSequence of NAT,
        i1,i2,i3,i4,i5,i6 for Element of n,
        a,b,c,d,e for Integer;

theorem Th3:
  k < m implies n+k in (n+m)
proof
  assume m>k;
  then k+n < n+m by XREAL_1:8;
  then n+k in Segm (n+m) by NAT_1:44;
  hence thesis;
end;
