GraphGroup/Template — pull request #3 by EmanueleMinotto (2013-01-17 23:07:19+00:00)
Title: Version 0.3.1

## Changelog
* defined the `Nodes` and `Arcs` interfaces
* manipulation using `map` methods
* defined the `Graph` interface as an ordered pair of `Nodes` and `Arcs`
* extraction and filtering methods
* shourtcuts