context sensitive read-only

Hello World, I would like to know how to change the read-only state of a buffer depending on the cursor position, in files where parts (lines) are surrounded by tags like: // {{{ USR tata yoyo ... // Inside tags, I need buffer writable (buffer-read-only = nil) ... // }}} USR tata yoyo Outside those parts, the buffer must be automatically set as read-only. Does a such mode exists, and if not, is it difficult to do ? Does mmm-mode can be used in this case ? Thanks for any answers ! Galanga