## #write-types different types of write operations

Git supplies enough information to the update hook to be able to distinguish
several types of writes.

The most common are:

  * `RW` -- create a ref or fast-forward push a ref.  No rewinds or deletes.
  * `RW+` -- create, fast-forward push, rewind push, or delete a ref.

Sometimes you want to allow people to push, but not *create* a ref.  Or
rewind, but not *delete* a ref.  The `C` and `D` qualifiers help here.

  * When a rule specifies `RWC` or `RW+C`, then *rules that do NOT have the C
    qualifier will no longer permit **creating** a ref*.

    <font color="gray">Please do not confuse this with the standalone `C`
    permission that allows someone to [create][] a **repo**</font>

  * When a rule specifies `RWD` or `RW+D`, then *rules that do NOT have the D
    qualifier will no longer permit **deleting** a ref*.

Note: These two can be combined, so you can have `RWCD` and `RW+CD` as well.

One very rare need is to reject merge commits (a commit series that is not a
straight line of commits).  The `M` qualifier helps here:

  * When a rule has `M` appended to the permissions, *rules that do NOT have
    it will reject a commit sequence that contains a merge commit* (i.e., they
    only accept a straight line series of commits).