fix the bug in the denv_delete_frac function. Really need to prove those functions.

theories/vcgen/denv.v

@@ -74,7 +74,7 @@ Section denv.
          ''(DenvItem lv q dv) ← dio;
          guard (lv = ULvl);
          Some (Some (DenvItem lv (q / 2) dv) :: m', (q / 2)%Qp, dv)
-      | S i => ''(m', q, dv) ← denv_delete_frac i m; Some (dio :: m', q, dv)
+      | S i => ''(mf, q, dv) ← denv_delete_frac i m'; Some (dio :: mf, q, dv)
       end
     end.