**Question.** How is the _nilpotent of class 2 (nil-2) free product_ of groups defined?

I came across this construction reading the following paper.


Alan H. Mekler (1981), Stability of nilpotent groups of class 2 and prime exponent. Journal of Symbolic Logic, 46, pp 781-788. doi:10.2307/2273227. 


There the author use the nil-2 free product in order to build
the nil-2 exponent $p$ group generated by some nice graph $\Gamma$.