Function find_parent 

pub fn find_parent<'a>(
    root: &'a Expression,
    target: &Expression,
) -> Option<&'a Expression>

Find the parent of target within the tree rooted at root.

Uses pointer identity (std::ptr::eq) — target must be a reference obtained from the same tree (e.g., via ExpressionWalk::find or DFS iteration).

Returns None if target is the root itself or is not found in the tree.