disabling unary "-" for unsigned types

[Walter Bright]

Steven Schveighoffer wrote:
> For example, there is no possible way a person unfamiliar with computers 

That's a valid argument if you're writing a spreadsheet program. But 
programmers should be familiar with computers, and most definitely 
should be familiar with 2's complement arithmetic.

Similarly, if you do much with floating point, you should be familiar 
with "What Every Computer Scientist Should Know About Floating-Point 
Arithmetic"

http://docs.sun.com/source/806-3568/ncg_goldberg.html