theorem Th26:
  rfs(r).(n+1) = 1 / (rfs(r).n-scf(r).n)
proof
  thus rfs(r).(n+1) = 1/frac(rfs(r).n) by Def3
    .= 1/(rfs(r).n-scf(r).n) by Def4;
end;
