Sentence:  If "f"("x") is a polynomial or a power series then its derivative can be obtained "automatically" using <WSD>dual numbers</WSD> as follows: f'(x) = {f(x + \epsilon) - f(x) \over \epsilon} where f(x + \epsilon) should be expanded using the algebra of <WSD>dual numbers</WSD>. The role of the "&epsilon;" is therein similar to that of an infinitesimal. 
Meaning of the word marked with <WSD> tags:  An element of an algebra (the "algebra of dual numbers") which includes the real numbers and an element &epsilon; which satisfies &epsilon; ≠ 0 and &epsilon;&sup2; = 0.