`M98` allows for essentially function calls (nested or not.)

If you had a part of a machine that could save state (say.. turning on a coolant pump..) I wonder how much more of a turing machine you could wrastle into it.

(or you could just cheat and use one of the hundreds of gcode variants that have computational stuff stapled into them like the Fanuc equivalents, but that's sorta dishonest for the exercise)