1. A computer-implemented method for modeling a target system, comprising: defining a conditional constraint satisfaction problem (CondCSP) that characterizes the target system in terms of problem variables and constraints applicable to the problem variables, the constraints comprising conditional constraints, such that the variables associated with the conditional constraints each have a respective activity status; defining existence variables to represent the respective activity status of the variables associated with each of the one or more conditional constraints; and finding a solution to the CondCSP by determining an assignment of the problem variables and the existence variables that satisfies the constraints.