theorem Th28:
  [\ r /] + i0 = [\ r + i0 /]
proof
  r - 1 < [\ r /] by Def6;
  then r - 1 + i0 < [\ r /] + i0 by XREAL_1:6;
  then
A1: r + i0 - 1 < [\ r /] + i0;
  [\ r /] <= r by Def6;
  then [\ r /] + i0 <= r + i0 by XREAL_1:6;
  hence thesis by A1,Def6;
end;
