Collection, why COW collection are important

[deadalnix via Digitalmars-d]

The hack team recently released project to add COW collection to 
hack in addition to current, reference type, collections. You can 
find explanation here :

http://hhvm.com/blog/10649/improving-arrays-in-hack

As collection are discussed here, I think this is relevant, 
especially the explanations as of why COW matters.