Objects from the Class
Objects can be created by calls of the form new("adjacencyList",id,pa,ne).Extends
Class "list", from data part.
Class "vector", by class "list".Warning
The pa, ch and ne constructor input lists are silently discarded
if their length differs from the actual number of vertices (determined by id). All input numeric identifiers
greater than this number are also silently discarded by the constructor. The id input to constructor is mandatory,
since the constructor needs to identify vertices. In addition, at least one between pa and ch should be present,
if directed edges have to be specified, and ne should be present, if undirected edges have to be specified.
If both pa and ch are present, the union of the two corresponding multi-sets of edges is specified.
If the neighbourhood relationship specified by ne is not symmetric, it is made symmetric by the constructor.