“…This consideration motivates the domain and range of K chosen above. Again, the above formulation of K is different to that typically used in the previous literature (see, e.g., [3,17,18]), but is again motivated by the desire to produce a precise and unambiguous definition of K, and to ensure that derivatives are only evaluated at points at which they are defined, for all classes of time scales (we observe that in, e.g., [3,18], the problem of evaluating derivatives at points outwith their normal domain of definition can again occur for certain time scales).…”