Correcting 'regression' after dumping some modifications to a dead end
branch (processing-order.) The useful lost fonctionality was the delayed
call to onBeg, which allows for more streamlined processors: they now
can put whatever code in onBeg and be ensured it is called once only,
neither too late, nor too soon. The previous implementation which called
onBeg on object creation prevented a smooth treatment for different
branches in inBeg depending on the object later state (i.e. a simple
leaf, or a node, or a list...)

Now, onInit plays the simple role of a pure initializer.