code: def _subsample(self):
          
          spans = self.maparr
          samp = np.zeros(spans.shape[0], dtype=np.uint64)
          for i in xrange(spans.shape[0]):
              samp[i] = np.random.randint(spans[i, 0], spans[i, 1], 1)
          return samp
docs: returns a subsample of unlinked snp sites