* lisp/dired.el (dired-mark-files-in-region,dired-mark):
A marked file can be remarked just:
1) To mark for deletion.
2) To unmark it, that is, the mark changes to `?\\s'.
3) If its marked for deletion, a different mark can be assigned.

The purpose of this changes is to prevent changing accidentally
the mark of a file. Current code allow to change marks on files very
easily.

Marking many files using several markers may need some time, with
some regexp gimnastics. Its a pity if someone accidentaly change
one of the marks. For instance, using `dired-mark-files-in-region',
the last/first file in the region could be included unintentionally
changing the mark of one file.


I'm not sure if allow 1) or to ban it.
I'm not sure if someone find this thread useful either :-)

In GNU Emacs 25.1.50.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.29)
  of 2016-03-03 built on calancha-pc
Repository revision: 887f6126c5ce9084f93083765ac026ca6b28175c