Projection Pursuit Classification Tree Extensions // Implements extensions to the projection pursuit tree algorithm for supervised classification, see Lee, Y. (2013), doi:10.1214/13-EJS810 and Lee, E-K. (2018) doi:10.18637/jss.v083.i08. The algorithm is changed in two ways: improving prediction boundaries by modifying the choice of split points-through class subsetting; and increasing flexibility by allowing multiple splits per group.