code: package edu.cmu.ml.rtw.pra.features;
  
  /**
   * Allows for some computation to be done for each vertex, instead of for each walk, if the
   * computation only depends on the vertex.  Classes that implement this interface will define their
   * own data structures and methods, and should cast to the subtype in the methods that use the
   * cache.
   *
   * One example of this is computing the distribution over edge types for a vector space walk - it
   * depends on the path type and the edges at the vertex, but not on the individual walk.
   */
  public interface PathTypeVertexCache { }
  