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I have written the article trying to avoid complications and trying to put together the different definitions of fluents in different formalisms. This caused some problems:
I have assumed that fluents are truth-valued for the sake of simplicity, but this is not always the case; I think the better solution is to add a separate paragraph rather than making the definition more general from the beginning;
I have used “the predicate ... ”; this is of course not a predicate but a literal using that predicate. Is there any way to say something correct without introducing the complication “we use a predicate so that ... ”?