def max_time_last
      max = 0
      i = 0
      while i < @children.size
        tl = @children[i].time_last
        max = tl if tl > max
        i += 1
      end
      max
    end
def max_time_last
      max = 0
      i = 0
      while i < @children.size
        tl = @children[i].time_last
        max = tl if tl > max
        i += 1
      end
      max
    end
Returns the maximum time last in all children

 @return [Numeric] the max time last