We present slight modifications to Fedi <2002>. In that paper EHD was defined by equation 2 and all the results there described were based on it. We here correct equation (3) and the previous one (unnumbered) of that paper. These equations refer to an alternative definition of EHD, based on the magnitude of horizontal derivatives, which is not equivalent to the EHD form of equation 2. Nothing of the paper is affected by these modifications, since it refer to equation 2 as a whole and the form (3) was not used at all.