> Suppose we have split declarations for derivations or defaults of
> symbols that are shared across architectures. It would then be
> locally convenient for configuration maintainers to move these
> declarations from the top-level rules.cml file piecemeal to rules
> files in different arch subt
> Because make on its own cannot cope with multiple source trees.
> Deleting an item from the higher source tree exposes the same item in a
> lower source tree, alas this item has an earlier timestamp. make only
> detects timestamps going forward, not backwards, with the result that
> the object
