If df/dx=f'(x sub0 = lim x-->0 f(x)-f(x sub 0)/x - x sub 0 = lim h --> 0 f(x sub 0 + h) - f(x sub 0)/h, defines a derivative, does it then follow that if the limits are defined as lim 0 --> x and lim 0 --> h, the definition is of a non-derivative?