The concepts described in articles in this category may be also expressed in terms of arguments, or rules of inference. Very often the same concept is in more than one of these categories, expressed a different way and sometimes with a different name. Please also observe that a theorem is distinct from a theory. A lemma is conceptually the same as a theorem. However a theorem is called a lemma when its proof is only considered to be a step in the proof of some other, more important theorem.

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