def add(value, nodeidentifier, edges=Array.new)
      node = ALNode.new(value)
      @nodes[nodeidentifier] = node
      @edges[nodeidentifier] = edges
      self
    end
def add(value, nodeidentifier, edges=Array.new)
      node = ALNode.new(value)
      @nodes[nodeidentifier] = node
      @edges[nodeidentifier] = edges
      self
    end
Returns a new AdjacencyList
 Nodes are accessed with unique names if +:named+ is true,
 otherwise they are accessed with integer indices (default).
 Assignment - adds a new node with +:value+, and
 +:nodeidentifier+, and optionally an array of
 identifiers of other nodes defining +:edges+.
 Returns self, so that assignments can be chained.