Jon Claerbout previously developed a two-dimensional finite-difference dip estimation scheme that turned out to be related to the three-dimensional one I discovered. Bee Bednar urged me to combine somehow two of Jon's operators to a three-dimensional discontinuity attribute and, thereby, prodded me into exploring the plane-wave estimation in greater detail than I first intended. Bill Symes pointed out the relationship between the cross-product operator and his Differential Semblance Optimization work.