def killCells(self, percent=0.05):
    """
    Changes the percentage of cells that are now considered dead. The first
    time you call this method a permutation list is set up. Calls change the
    number of cells considered dead.
    """
    numColumns = numpy.prod(self.getColumnDimensions())

    if self.zombiePermutation is None:
      self.zombiePermutation = numpy.random.permutation(numColumns)

    self.numDead = int(round(percent * numColumns))

    if self.numDead > 0:
      self.deadCols = self.zombiePermutation[0:self.numDead]
    else:
      self.deadCols = numpy.array([])

    self.deadColumnInputSpan = self.getConnectedSpan(self.deadCols)
    self.removeDeadColumns()
Changes the percentage of cells that are now considered dead. The first
    time you call this method a permutation list is set up. Calls change the
    number of cells considered dead.