reserve a,b,p,k,l,m,n,s,h,i,j,t,i1,i2 for natural Number;

theorem
  n < k implies k -' (n+1) + 1 = k -' n
proof
  assume
A1: n < k;
A2: k -' n = k - n by A1,XREAL_1:233;
  n+1 <= k by A1,NAT_1:13;
  then k -' (n+1) = k - (n+1) by XREAL_1:233;
  hence thesis by A2;
end;
