The paper describes a smart interface that makes numerical libraries easy to use and reliable by exploiting mathematical theory, symbolic algebra, and descriptions of the input/output formats of numerical subroutines. The interface accepts a high-level problem description, selects the appropriate numerical subroutine, programs the problem in subroutine format, runs the program, corrects for numerical errors and special conditions, and returns the output in a high-level format. The current interface manages a root finder, a continuation package, an ordinary differential equation integrator, and a Lyapunov exponent calculator. I describe these functions and illustrate their use in a program that analyzes ordinary differential equations.