# What is the Pacman lemma?

I've seen "the Pacman lemma" mentioned in the context of reduction orderings on logical terms, but a Google search doesn't find a definition; what exactly is it? Closest a search found was a "Pacman rule" in the context of calculus; is that the same thing?

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Lemma. : Let $(X,\mathcal{S})$ be a measurable space, $\nu$ a signed measure. Suppose we have a subset $E \in \mathcal{S}$ and $0 < \nu(E) < \infty$ . Then, there exists a positive set $P \subset E$ such that $\nu(P) > 0$.