[taradinoc]

Yeah, if there's nothing after the final !- in an atom, it means the atom is in <ROOT>. (See section 15.5 in the manual.)

I'm not sure what the purpose of things like <RETURN!-> and <ORB!-> is. It could have something to do with the way they implemented Z-machine opcodes as MDL atoms, i.e. <RETURN!-> forces it to use the MDL SUBR instead of the opcode? Maybe they moved the MDL atoms that collided with Z-machine opcodes to a separate OBLIST, and this was needed to access them from MDL code in the same file?

I never figured that part out, so when ZILF is generating code, it uses the same OBLISTs it used while interpreting and just looks things up by their PNAMEs.

SETG20 and DEFINE20 are used in "MDL-ZIL" files, where routines are defined with DEFINE instead of ROUTINE, global variables are created with SETG instead of GLOBAL, etc. Presumably that was a way to run the games in MDL during development to avoid recompiling them. SETG20 and DEFINE20 are aliases for the MDL versions of SETG and DEFINE.