+ "Display message file with index DUPLICATE in place of the current one.
+
+Message file indices are based on the order the files are
+discovered by `notmuch new' (and hence are somewhat arbitrary),
+and correspond to those passed to the \"\\-\\-duplicate\" arguments
+to the CLI.
+
+When called interactively, the function will prompt for the index
+of the file to display. An error will be signaled if the index
+is out of range."