As a result a Unit is a special kind of object, it can not be a subject .
In a computer the Unit is a bit value. This is the smallest Unit with which current computers can work.
In XML the Unit is a UNICODE character. This is the smallest Unit with which XML aware applications can work (e.g. parsers, translators, ...)
Note:
In XML you can have external entities which are not based on UNICODE, but as the name implies these are external to XML.
Current Mathematics works with 2 Units. The numbers 0 and 1. All other mathematic numbers (which are actually expressions) can be written as a sequence of functions on these numbers.
In Compositional Mathematics there is only 1 Unit. Everything else is a composition.
A complete description of Compositional mathematics will be available in the forthcoming book "Compositions" by Koen Van Exem.