...as a prefix. (See: ternary operators, named parameters)
I still think `:` is a good option...

...is now that we have named function parameters it might result in a bit of...

...not function pointer/delegate types that are parameters or return types.
// For `callback`, the module...

...a message saying why that combination of parameters couldn't be passed to that function...

https://issues.dlang.org/show_bug.cgi?id=24635

Bolpat <qs.il.paperinik@gmail.com...

...ID: 24635
           Summary: Allow opApply with default parameters
           Product: D
           Version: D2
          Hardware: All
                OS...

...we need.

We could instead introduce named parameters which can only be set via a...

Let function and template parameters carry optionally a visibility attribute. For example:
```d
void log...

https://issues.dlang.org/show_bug.cgi?id=24629

Bolpat <qs.il.paperinik@gmail.com...

...Allow more than 1 set of template parameters
           Product: D
           Version: D2
          Hardware: All
                OS...