What does ! Stand for in map! and filter! function calls?

[Adam D. Ruppe]

On Sunday, 2 June 2019 at 16:39:57 UTC, Rnd wrote:
> Is it not possible in the language to have template map 
> function also to called as map(x=>... ?

It could be done pretty easily, but you will have to learn what 
template arguments are eventually anyway. You'll see this pattern 
all over, like `to!string(...)` among others that come up pretty 
early using D.