  Another way of thinking about hypersets is in terms of systems of set equations, as we have done it for streams and trees.  By such a system we mean a set X which we think of as variables (any set will do), and then a function e from X to its power set  ℘X.  That is, the value of e on each variable is again a set of variables.  Set systems and related concepts correspond to ones for graphs in the following way: