diff --git a/lib/command.h b/lib/command.h
index 8fdec98..eba919c 100644
--- a/lib/command.h
+++ b/lib/command.h
@@ -322,7 +322,11 @@
 void install_element (enum node_type, struct cmd_element *);
 void sort_node ();
 
-char *argv_concat (const char **, int, int);
+/* Concatenates argv[shift] through argv[argc-1] into a single NUL-terminated
+   string with a space between each element (allocated using
+   XMALLOC(MTYPE_TMP)).  Returns NULL if shift >= argc. */
+char *argv_concat (const char **argv, int argc, int shift);
+
 vector cmd_make_strvec (const char *);
 void cmd_free_strvec (vector);
 vector cmd_describe_command ();