dep_prev: Make later chunks depend on previous chunks
Description
This function can be used to build dependencies among
  chunks so that all later chunks depend on previous
  chunks, i.e. whenever the cache of a previous chunk is
  updated, the cache of all its later chunks will be
  updated.Value
- NULL; the internal dependency structure is updated
  as a side effect.
References
http://yihui.name/knitr/demo/cache/