I would like to programmatically evaluate all cells in a Notebook except
those in the last CellGroup. Assuming the Notebook is open and has name
"foo", this code should work:
   (*1*) nb=Notebooks["foo"]//First;
   (*2*) SelectionMove[nb,All,Notebook];
   (*3*) UnselectLastCellGroup[nb];
   (*4*) SelectionEvaluate[nb];
Unfortunately, 'UnselectLastCellGroup' is pseudo code; I do not know how to
implement it although it seems like the pair of steps (*1*) and (*2*) ought
to be functionally equivalent to some sequence of calls to SelectionMove.

Some related questions:
1. Given a NotebookObject nb, how to determine how many cells it contains?
2. Is a CellGroup also a cell? (I am contemplating stepping through a
notebook cell by cell.)

Any and all advice appreciated!