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A widely used clause in license contracts - the field-of-use restriction (FOUR) - precludes licensees from operating outside of the specified technical field. When a technology has several distinct applications, FOUR allow the licensor to slice up his rights and attribute them to the lowest-cost producer in each field of use. This can improve production efficiency. With complex technologies, however, the boundaries of fields of use may be difficult to codify, entailing a risk of overlap of licensees' rights. We explore how this affects the optimal license contract when the licensor's effort determines the probability of overlap.