-- Examples:
--
-- Atkinson, p. 60.
+--
-- >>> let f x = x^6 - x - 1
-- >>> let f' x = 6*x^5 - 1
-- >>> tail $ take 4 $ newton_iterations f f' 2
-- Examples:
--
-- Atkinson, p. 67.
+--
-- >>> let f x = x^6 - x - 1
-- >>> take 4 $ secant_iterations f 2 1
-- [2.0,1.0,1.0161290322580645,1.190577768676638]
-- Examples:
--
-- Atkinson, p. 67.
+--
-- >>> let f x = x^6 - x - 1
-- >>> let Just root = secant_method f (1/10^9) 2 1
-- >>> root