The Attentive Reader
Definition. The Attentive Reader is a quasi-mythical being whose existence has been postulated, and often tacitly assumed, by textbook writers. Citations show that the Attentive Reader has, in addition to an eidetic memory for the work itself, an infinite attention span, and an uncanny knack for predicting and rationalising the wild turns of the author's arguments.
Usage example: The Attentive Reader will have realised there is a microscopic disparity between the conclusion of Lemma 1.2.3 and the hypothesis of Proposition 153.1.23. This can be explained by an appeal to the Robinson compactness principle (as modified by Heimholtz).