Description: In a Moore system, a set isindependent if no element of the set is in the closure of the set with the element removed (Section 0.6 in Gratzer p. 27; Definition 4.1.1 in FaureFrolicher p. 83.) mrInd is a class function which takes a Moore system to its set of independent sets. (Contributed by David Moews, 1-May-2017)