"fold": a replacement for "reduce"

[Timon Gehr]

On 03/27/2014 07:04 PM, Meta wrote:
>
> It's a bit unintuitive as to what works and what doesn't (although it
> makes sense for the most part).

It doesn't make sense at all. It is an arbitrary limitation. The rule is 
simple though: One can only alias things that syntactically look like 
they might be types. This is why the following triviality is way more 
useful than it should be:

alias Id(alias a)=a;

alias fun = Id!(a=>a); // ok!