In this example how how to use the library, we work on very simple node types, which only have an header and a value for it (i.e., a single TabValueGroup with empty tail). Moreover, nodes are defined as a straight extension of the DefaultNode in the library. This is just an example, typically you rather prefer to define wrappers for your object model (See simple_biomodel_tests). Note that the graphs here are 'dummy' only as models and from the biological standpoint. From the topological point of view, you'll find rather complex structures put here to test graph2tab.