A computer-implemented method for modeling a target system includes 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 include conditional constraints, such that the variables associated with the conditional constraints each have a respective activity status. Existence variables are defined to represent the respective activity status of the variables associated with each of the one or more conditional constraints. A solution to the CondCSP is found by determining an assignment of the problem variables and the existence variables that satisfies the constraints.