MDL (pronounced “muddle”; a backronym for Model Development Language) is a Lisp-descended programming language developed at MIT in the 1970s. It was used to develop the initial version of the game Zork.
On twenex.org, run the MDL interpreter with the command MDL105:
[KANKAN] PUBLIC:<~>@ MDL105 MUDDLE 105 IN OPERATION. LISTENING-AT-LEVEL 1 PROCESS 1
Quit MDL by pressing Ctrl-C.
After typing an expression in the interpreter, press the Esc key (represented with $ in manuals; used instead of the more common Enter key) to have MDL process what you've typed.
Create source files with any editor. Save the file with the extension “.MUD”.
Load source files into the MDL interpreter with the function FLOAD:
<FLOAD "DEV:<DIR>FILE">
(You can omit the DEV:<DIR> portion of the file specification if you set the MDL: logical name to the device and directory where your MDL source files are stored.)