about negative keyword in VHDL

[Dudhat M. (Firma: student) (dudhat01)]

hiii,

could anybody tell me about the negative number keyword...?
i.e..
let us say if we say a > -b..so can we write a > neg(b)...????

or..

[Andreas U. (Gast)]

Dudhat, this is the third or forth question I read from you, which 
easily could be answered by a colleague or are not well defined to 
answer.

This in particular can be read from any FPGA introduction.