def node_children(node)
      children = @navigator.call(node)
      return Array::EMPTY_ARRAY if children.nil?
      Enumerable === children ? children.to_a.compact : [children]
    end
def node_children(node)
      children = @navigator.call(node)
      return Array::EMPTY_ARRAY if children.nil?
      Enumerable === children ? children.to_a.compact : [children]
    end
Returns the children to visit for the given node.