The fundamental bigroupoid $\Pi_2(X)$ of a space $X$ was independently described by [Hardie, Kamps and Kieboom](http://dx.doi.org/10.1023%2fA%3a1011270417127) (paywall) and [Stevenson](http://arxiv.org/abs/math/0004117) (arXiv) around the year 2000. HKK cite Baez-Dolan's seminal [HDA0](http://arxiv.org/abs/q-alg/9503002), but the idea there is just very roughly sketched, and cited Grothendieck's [Pursuing Stacks](http://ncatlab.org/nlab/show/Pursuing+Stacks) for inspiration. The idea goes back to a letter in February 1975 from Grothendieck to Breen describing the ideas later expanded in Pursuing Stacks:

![Grothendieck's Feb 1975 letter to Breen](https://i.sstatic.net/ydOwg.png)

However, I would be surprised if the notion wasn't some kind of folklore. What other sources apart from those I've cited mention $\Pi_2(X)$?

Bénabou doesn't mention bigroupoids in his paper introducing bicategories, so a subsidiary question is: when were bigroupoids were first mentioned?