Square Rectangular Mail

From the new postal guidelines:

All large envelopes (flats) will be required to be rectangular in shape. This includes square pieces.

This had triggered jokes such as -- wow, if a square is not rectangular, they cannot deliver it?

While this is somewhat bad and unclear writing, there is of course a straightforward explanation -- it is a ribbuy (inclusion) rather than a miut (exclusion). All large envelopes (flats) will be required to be rectangular in shape. Presumably for machines to be able to process better. Ah, but what is the definition of a rectangle? There is a lenient mathematical definition, under which squares are a type of rectangle. There is also a more stringent colloquial definition under which a rectangle is a shape in which the length is not equal to the width. And perhaps the machines might even have difficulty processing this. Thus, they say that the set "rectangular in shape" "includes square pieces."