Function children 

pub fn children(c: &Core) -> Vec<&Core>

Every subexpression of this one.

The read-only twin of children_mut, and it exists for the same reason: a pass that walks the whole tree should not restate the shape of CoreKind, because the day a variant gains a child every hand-written walk is silently incomplete (docs/90 §90.5 is that failure, at fourteen sites).