ungrouping certain hierarchies (RTLC loop issue)

I am ungrouping certain hierarchies in my design. I have written a small tcl in RTLC to do so:



Suppose I have a design $Top, having hierarchy as follow:

$Top

- A (Inst name: i_A_inst)

- B (Inst name: i_B_inst)

- C (Inst name: i_C_inst)

- D (Inst name: i_D_inst)

- E (Inst name: i_E_inst)

- F (Inst name: i_F_inst)

- G (Inst name: i_G_inst)



### List of subdesigns to be preserved from ungrouping

set keep_hier_list [list A B C D]



### Preserve all the subdesigns to be prevented from ungrouping

foreach ele $keep_hier_list {

set_attr preserve true [find –subdesign $ele]

}



### Ungroup all instances except for those preserved as above

Ungroup –flatten /designs/$Top/instances_hier/*

Warning : Cannot ungroup preserved instance. [UTUI-100]

: /designs/$Top/instances_hier/i_A_inst

Warning : Cannot ungroup preserved instance. [UTUI-100]

: /designs/$Top/instances_hier/i_C_inst

Warning : Cannot ungroup preserved instance. [UTUI-100]

: /designs/$Top/instances_hier/i_E_inst



### Ungroup all the instances inside preserved modules except for some preserved ones as in loop 1

foreach ele $keep_hier_list {

set_attr preserve false [find –subdesign $ele]

set inst_name [get_attr instance $ele]

ungroup –flatten $inst_name/instances_hier/*

set_attr preserve true [find –subdesign $ele]

}





### Removing preserve from these subdesigns to proceed with further optimizations

foreach ele $keep_hier_list {

set_attr preserve false

}





Now, when loop 2 is executed all the instances inside A except B will be ungrouped and for B it will give the following warning and come out of loop:

Warning : Cannot ungroup preserved instance. [UTUI-100]

: /designs/$Top/instances_hier/i_A_inst/instances_hier/i_B_inst



But this is intentional because I want to preserve subdesign B (or instance i_B_inst), and I want to go to second iteration for this loop for next element.

How can I do this?

Thanks