ne's manual

4.9.8: ReadOnly

performed on the document (any such attempt produces an error message). This flag is automatically set whenever you open a file which you cannot write to. See Open.

If you call this command with no arguments, it will toggle the flag. If you specify 0 or 1, the flag will be set to false or true, respectively. A lower case `r' will appear on the status bar if the flag is true.