Function single_flush 

pub const fn single_flush(prefix: usize) -> bool

Whether prefix keeps the 68th node from overflowing into a second word.

before_last is the byte offset where node 67 starts inside its word. If node 67 fits completely in that word, the stream ends with exactly one partial word and the single output register is enough. The prefix must also be even and shorter than one word (assumptions the packer relies on).