Searching the internet, it seems that it existed for decades (1993, H. J. Carmichael), it simply wasn't popular: https://journals.aps.org/prl/abstract/10.1103/

Searching the internet, it seems that it existed for decades (1993, H. J. Carmichael), it simply wasn't popular: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.70.2273 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.70... and the 1999 paper: https://link.springer.com/chapter/10.1007/BFb0105336 https://link.springer.com/chapter/10.1007/BFb0105336