array[0:0] should return empty array

[FilipeMaia]

Currently it is equivalent to array[:] which is quite surprising.

[FilipeMaia]

As empty arrays don't really exist yet a good compromise might to to return None.

[pavanky]

They do exist. array(dim4()) is an empty array in ArrayFire. The functions not handling them properly is a different issue..

[pavanky]

This is partially solved when using logical indexing in devel.