continued_fraction_quadratic does reduce to continued_fraction_rational on iterators with no repeating portion (no final element that is a list).  Should I try to combine the two into one function?  What would be a good name for it?  Most of the time people will just be wanting to reduce a normal terminating continued fraction back into its simple `p/q` form, so the quadratic name could be confusing, but it's confusing in the other direction without it.