Where are the strings in std.string?

[TheManWithNoName]

I don't mean to be splitting hairs here, well maybe I do, but

D clearly makes the string type to be immutable(char)[].  That's 
fine, but the std.string is all about char[] or mutable character 
arrays.

So now, whenever I see a reference to the string, I have to ask 
myself: do they mean the formal definition of string, 
immutable(char)[], or the colloquial version, char[].