code: def _find_bck(self, chunk):
          
          cur = self.free_head_chunk
          if cur is None:
              return None
          fwd = cur.fwd_chunk()
          if cur == fwd:
              return cur
          # At this point there should be at least two free chunks in the heap
          if cur < chunk:
              while cur < fwd < chunk:
                  cur = fwd
                  fwd = cur.fwd_chunk()
              return cur
          else:
              while fwd != self.free_head_chunk:
                  cur = fwd
                  fwd = cur.fwd_chunk()
              return cur
docs: Simply finds the free chunk that would be the backwards chunk relative to the chunk at ptr. Hence, the free head
          and all other metadata are unaltered by this function.