We first use locateXO to identify crossover locations.
  If a pair of adjacted crossovers are separated by no more than
  maxdist and contain no more than maxmark genotyped
  markers, the intervening genotypes are omitted (that is, changed to
  NA).
The arguments maxdist and maxmark may be vectors.  (If
  both have length greater than 1, they must have the same length.)  If
  they are vectors, genotypes are omitted if they satisify any one of
  the (maxdist, maxmark) pairs.