package edu.stanford.nlp.optimization;/** * An interface for once-differentiable double-valued functions over * double arrays.  NOTE: it'd be good to have an AbstractDiffFunction * that wrapped a Function with a finite-difference approximation. * * @author <a href="mailto:klein@cs.stanford.edu">Dan Klein</a> * @version 1.0 * @since 1.0 * @see Function */public interface DiffFunction extends Function {  /**   * Returns the first-derivative vector at the input location.   *   * @param x a <code>double[]</code> input vector   * @return the vector of first partial derivatives.   */  double[] derivativeAt(double[] x);}