def index( child )
      count = -1
      @children.find { |i| count += 1 ; i.hash == child.hash }
      count
    end
def index( child )
      count = -1
      @children.find { |i| count += 1 ; i.hash == child.hash }
      count
    end
Fetches the index of a given child
 @param child the child to get the index of
 @return the index of the child, or nil if the object is not a child
 of this parent.