Current Pset and Variable Psets

" Defining Psets" describes how to create variable psets--user-defined psets whose membership can change in the course of program execution. You use the eval pset command to update the membership of a variable pset. If you make a variable pset your current set, its membership is determined by the most recent eval pset command you have executed for the set. If you have not executed an eval pset command to update the set's membership, the membership continues to be what it was when you created the set.