In the systems described so far, it has been assumed that the occupation of a site or bond is completely random—this is the so-called "Bernoulli percolation." For a continuum system, random occupancy corresponds to the points being placed by a Poisson process. Further variations involve correlated percolation, such as percolation clusters related to Ising and Potts models of ferromagnets, in which the bonds are put down by the Fortuin-Kasteleyn method. In "bootstrap" or "k-sat" percolation, sites and/or bonds are first occupied and then successively culled from a system if a site does not have at least "k" neighbors. Another important model of percolation, in a different universality class altogether, is directed percolation, where connectivity along a bond depends upon the direction of the flow.