TO-DO LIST:

-- 	Raise an error of an unknown attribute type. Probably, once we
	finish parsing a Model, we have to iterate through non-resolved
	attributes in the symbol table. 

Possibilities for improvement:

0) In the "default" virtual machine create a default meta-model, made of
a Node with name "Node", an Edge with name "Edge", etc. Whose types are
themselves. This will allow a uniform handling of types.

1) In Models: we should devise a mechanism to, at the meta-model level, allow 
or forbid submodels, and to define which elements are permitted in these submodels. By
now, we can always include submodels.

2) In Compiled Models: be able to get elements with getNode and getEdge (in addition
to use the tailored interface...)