Nilpotent Lazard-divided Lie ring

Definition
A Lazard-divided Lie ring $$L$$ is termed nilpotent of nilpotency class a nonnegative integer $$c$$ if $$\gamma_{c+1}(L) = 0$$, where $$\gamma_{c+1}(L)$$ denotes the $$(c+1)^{th}$$ member of the lower central series of $$L$$.