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

theorem
  i <= j implies j-'(j-'i) = i
proof
  assume
A1: i <= j;
  j-'(j-'i) + (j-'i) = j by Th50,XREAL_1:235
    .= i + (j-'i) by A1,XREAL_1:235;
  hence thesis;
end;
