How to read this signature? (writeTo in DIP9)

[Graham Fawcett]

Hi folks,

I'm having trouble deciphering this function signature, which appears
in the DIP9 proposal:

void writeTo(scope delegate(in char[] data) sink, 
             string format = null) const

// from http://prowiki.org/wiki4d/wiki.cgi?LanguageDevel/DIPs/DIP9

Particularly, how does "scope" modify the delegate, and what is the
meaning of the trailing "const"?

Thanks,
Graham