Improve "Improve Contract Syntax" DIP 1009

[Mark]

On Friday, 3 November 2017 at 02:32:41 UTC, Jonathan M Davis 
wrote:
> Pretty much the only case where out contracts work well is when 
> you have a very specific, testable condition that all results 
> must have and which does not depend on the input, and most 
> functions simply don't work that way.
> - Jonathan M Davis

Can you give an example of a function for which this doesn't work?