Constant APPEND 

pub const APPEND: u8 = _; // 64u8

Ask fs for an append-only capability: writes through it may only move FORWARD.

A flag in fs’s OPEN request, not a kernel right - the kernel’s rights are bits 0-5, and this bit never reaches it: fs mints an ordinary WRITE capability and records the restriction against the resource. Do not pass it where a kernel right is expected.

Enforced by fs against a HIGH-WATER MARK - the furthest offset written through this capability - not against the file’s size: a write below the mark is refused (Error::PermissionDenied). The first write may land anywhere, so this does not protect content that existed before the open, and a fresh open starts a fresh mark.