[RFA 1/4] Implement user defined prefix.

[Eli Zaretskii]

> From: Philippe Waroquiers <philippe.waroquiers@skynet.be>
> Cc: Philippe Waroquiers <philippe.waroquiers@skynet.be>
> Date: Sun, 29 Sep 2019 22:54:24 +0200
> 
> +  set_cmd_completer (define_cmd_element, command_completer);
> +  c = add_com ("prefix-define", class_support, prefix_define_command,
> +	   _("\
> +Define or mark a command as a user-defined prefix command.\n\
> +User defined prefix commands can be used as prefix commands for\n\
> +other user defined commands.\n\

I suggest to use "user-defined" consistently, with a hyphen.