how can i get a sub-tree(between two chapter)

[abbouh <ab...@ra...>]

i have this structure in my input file:

docbody|---chap1
                           |---ht
                           |---(para|list|notesect)*
                           |---chap2
                           |              |---ht
                           |              |---(para|list|notesect)*
                           |              |---chap3
                           |              |             |---ht
                           |              |
|---(para|list|notesect)*
                           |              |
                           |              | ---chap3
                           |              |             |---ht
                           |              |
|---(para|list|notesect)*
                           |              |
                           |              |---chap3
                           |              |             |---ht
                           |              |
|---(para|list|notesect)*
                           |
                           |---chap2
                                         |---ht
                                         |---(para|list|notesect)*
                                         |---chap3
                                         |             |---ht
                                         |
|---(para|list|notesect)*
                                         |
                                         | ---chap3
                                         |             |---ht
                                         |
|---(para|list|notesect)*
                                         |
                                         |---chap3
                                         |             |---ht
                                         |
|---(para|list|notesect)*


and i want for each chapter get its length,so how can i get a
sub-tree(between two chapter) in this file like:

-for chapter1:

chap1
         |---ht
         |---(para|list|notesect)*

-for chapter2:

 |---chap2
             |---ht
             |---(para|list|notesect)*


-for chapter3:

 |---chap3
             |---ht
             |---(para|list|notesect)*

thanks.