Finding position of a value in an array

[JN]

On Sunday, 29 December 2019 at 08:31:13 UTC, mipri wrote:
>
> int i = a.countUntil!(v => v == 55);
> assert(i == 2);

I also had to ask because I couldn't find it. In other languages 
it's named "index()", "indexOf()" or "find()". D is the only 
language I know which uses the "countUntil" scheme. And even so 
it's not obvious from the name if it's the index of the element 
or number of preceding elements.