W3-MathML questions: real/float, annotation, etc.

Hi,

while implementing MathML support for ConTeXt
(http://www.w3.org/Math/#Software) we came across of something which
seemed not to be very clear to us:

a) It is not clear what the difference between "float" and "real.
(I cannot find "float" anymore, maybe the PDF file is/was older)
Also the difference between "complex" and "complex-cartesian" is not
clear. (I guess there is none.)

b) Are there any guidelines which <semantic/> presentations should be
favoured and if it is allowed to have both xml-annotations MathML-Content
and/or MathML-Presentation (the examples use only the latter).

c) Do you know a list of the different presentations of mathematical
operators (such as "n over k" which I cannot find in the HTML version).

With warm regards,

Tobias