+Optional arg REGION, if non-nil, mean restrict search to the
+specified region. Otherwise search the entire buffer.
+When REGION is non-nil, it must be a cons (START . END).
When someone decides to add support for rectangular regions in occur later,
your current implementation will make this problematic since it changes
the format ((START . END)) to (START . END), that makes difficult to support
((START1 . END1) (START2 . END2) ...) later.
Let's stick to the same format in all uses of the new ‘REGION’ arg, and
currently in occur support only the degenerate case of ((START . END))
for non-rectangular regions.