Some mathematicians have emphasized that the process of getting a concept right in the degenerate case often leads one to discover and formulate the right collection of ideas for handling the general case in a robust manner. When a theory needs explicitly to exclude the empty set or some other trivial case—the disease known as “emptysetitis”—it is a sign that one has not yet found the right formulation. For example, is the empty topological space connected? Is the empty graph a finite connected planar graph? If so, it would contradict Euler’s theorem that v − e + f = 2 for finite connected
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