area1 
                   BFR1a BFR1b
                     /    \
        ....................................
        .                Core              .
        ....................................
        |    /       \    /           \  |
      BFR2a BFR2b  BFR3a BFR3b      BFR6a BFR6b
       /-------\   /---------\      /--------\
       | area2 |   |  area3  | ...  | area6  |
       | ring  |   |  ring   |      | ring   |
       \-------/   \---------/      \--------/
       more BFRs    more BFRs        more BFRs