yamaha FM synths of the sy/tg 77 and sy99 generation had a "freemform algorithm" , which could be configured by sysex only. it was possible, because they had arranged their FM engine in a bus-like system, where then every operator was able to send and receive from every other. i´ll attach a pic from the editor software "FM alive", as thats easier to understand than words. the downside of this approach is, that carriers sending to the audio(output) bus were scaled automatically in volume to avoid clipping. this created unwanted problems, if one used them also as modulators simultaneously.