Collection, why COW collection are important

[Ola Fosheim Grøstad via Digitalmars-d]

On Sunday, 1 November 2015 at 13:43:25 UTC, Gary Willoughby wrote:
> What does COW mean here?

Copy-On-Write. You can use it with shared ref counting and 
immutable for filling out templates, transactional computations 
and other situations where you want to create  a new version of 
something without affecting the original immediately.