Module top                                  top
      Cell b0
    Module buff                               top.b0
         input buf_in
         inoutput out
        Cell i0
         .in(buf_in)
         .out(a)
      Module inv                              top.b0.i0
           input in
           output out
        Cell i1
         .in(a)
         .out(buf_out)
      Module inv                              top.b0.i1
           input in