+/** Find a real root of real-valued function f(x) numerically within a given
+ * interval. The function must change sign across interval. Uses Newton-
+ * Raphson method combined with bisection in order to guarantee convergence.
+ *
+ * @param f Function f(x)
+ * @param x Symbol f(x)
+ * @param x1 lower interval limit
+ * @param x2 upper interval limit
+ * @exception runtime_error (if interval is invalid). */
+const numeric fsolve(const ex& f, const symbol& x, const numeric& x1, const numeric& x2);
+