S = PAIRDIST(X,Y,'safe') indicates that it is possible for
   the ith row of X to be identical to the jth row of Y.  This uses a
   slower algorithm, but guarantees that DISTS(i,j) == 0 in these cases.
   For the special case of PAIRDIST(X), it is not necessary to specify
   the 'safe' flag if the only potential zero distances are those along
   the diagonal of DISTS.

   DISTS = PAIRDIST(..., 'nosqrt') returns DISTS as above, except that
   the squared distances are returned.  The algorithm to compute the
