-If it is a string then that string should be an external program
-which must take a single argument (searched string) and output a
-list of completion candidates, one per line.
+If this is a string, then that string should be an external
+program, which must take a single argument (searched string)
+and output a list of completion candidates, one per line.
+
+If this is the symbol `internal', then an implementation is used
+that relies on the \"notmuch address\" command, but does not use
+any third-party (i.e. \"external\") programs.