Exercise 3.62: Use the results of Exercise 3.60 
and Exercise 3.61 to define a procedure div-series that
divides two power series.  Div-series should work for any two series,
provided that the denominator series begins with a nonzero constant term.  (If
the denominator has a zero constant term, then div-series should signal
an error.)  Show how to use div-series together with the result of
Exercise 3.59 to generate the power series for tangent.