module Net::IMAP::SASL::GS2Header

binding must override this method.
The default always returns "+n+". A mechanism that supports channel

The selected channel binding follows "+p=+".
"+p+":: The client requires channel binding.
but thinks the server does not.
"+y+":: The client does support channel binding
"+n+":: The client doesn't support channel binding.

+gs2-cb-flag+:
The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]

def gs2_cb_flag; "n" end