File permission confusion [Debian 9.1 with MATE]
As user "richard" I created 3 files.
I later wanted to protect them totally from accidental change.
For each file, I went to Properties->Permissions and changed Access for
Owner, Group, and Others to "Read Only".
As user "richard" I was able to delete them with Caja.
As "root" I changed Owner and Group to "root" leaving Access for all as
User "richard" could still *DELETE THEM*!
"Read Only" evidently does not mean what it implies.