Description: Define the derivative operator. This acts on functions to produce a
function that is defined where the original function is differentiable,
with value the derivative of the function at these points. The set
s here is the ambient topological space under which we are
evaluating the continuity of the difference quotient. Although the
definition is valid for any subset of CC and is well-behaved when
s contains no isolated points, we will restrict our attention to the
cases s = RR or s = CC for the majority of the development,
these corresponding respectively to real and complex differentiation.
(Contributed by Mario Carneiro, 7-Aug-2014)