Sequences of commutator operations
  Given the congruence lattice L of a finite algebra A with a Mal'cev term, we look for those sequences of operations on L that are sequences of higher commutator operations of expansions of A.   The properties of higher commutators proved so far delimit the number of such sequences: the number is always at most countably infinite; if it is infinite, then L is the union of two proper subintervals with nonempty intersection. 
Erhard Aichinger and Nebojsa Mudrinski
math.RA