Can someone lead me to any documentation or simple examples, about ATL Higher Order Transformations and the use of the complete ATL metamodel? There is already a link in older similar topic but it doesn't work.
It defines ATL's module superimposition semantics in an ATL HOT by feeding ATLCopy.atl and Superimpose.atl into themselves, resulting in ATLSuperimpose.atl. You can start by looking at ATLCopy.atl, which just creates an exact copy of an ATL transformation.
N.B. Things have become slightly easier for HOTs since EMFTVM, where you no longer need to explicitly inject .atl files into .xmi files (you can just load them as regular models). Also, refining mode can now be used to modify ATL models in-place instead of having to copy everything.