A válaszom mindjárt ott van az első linkeden:

The difference between a circular definition and a recursive definition is that a recursive definition must always have base cases, cases that satisfy the definition without being defined in terms of the definition itself, and all other cases comprising the definition must be "smaller" (closer to those base cases that terminate the recursion) in some sense. In contrast, a circular definition may have no base case, and define the value of a function in terms of that value itself, rather than on other values of the function. 

Ha pedig a '(X -> Y) = X' szerinted axióma, attól még éppúgy visszautasíthatom az elfogadását.