Street, R., & Walters, R. (1978). Yoneda structures on 2-categories. Journal of Algebra, 50(2), 350–379. https://doi.org/10.1016/0021-8693(78)90160-6 
@article{Street1978,
  title = {Yoneda structures on 2-categories},
  author = {Street, Ross and Walters, Robert},
  year = {1978},
  month = feb,
  journal = {Journal of Algebra},
  publisher = {Elsevier {BV}},
  volume = {50},
  number = {2},
  pages = {350--379},
  doi = {10.1016/0021-8693(78)90160-6},
  url = {https://doi.org/10.1016/0021-8693(78)90160-6}
}