def append_leaves(
    new_line: Line, old_line: Line, leaves: list[Leaf], preformatted: bool = False
) -> None:
    """
    Append leaves (taken from @old_line) to @new_line, making sure to fix the
    underlying Node structure where appropriate.

    All of the leaves in @leaves are duplicated. The duplicates are then
    appended to @new_line and used to replace their originals in the underlying
    Node structure. Any comments attached to the old leaves are reattached to
    the new leaves.

    Pre-conditions:
        set(@leaves) is a subset of set(@old_line.leaves).
    """
    for old_leaf in leaves:
        new_leaf = Leaf(old_leaf.type, old_leaf.value)
        replace_child(old_leaf, new_leaf)
        new_line.append(new_leaf, preformatted=preformatted)

        for comment_leaf in old_line.comments_after(old_leaf):
            new_line.append(comment_leaf, preformatted=True)

    Append leaves (taken from @old_line) to @new_line, making sure to fix the
    underlying Node structure where appropriate.

    All of the leaves in @leaves are duplicated. The duplicates are then
    appended to @new_line and used to replace their originals in the underlying
    Node structure. Any comments attached to the old leaves are reattached to
    the new leaves.

    Pre-conditions:
        set(@leaves) is a subset of set(@old_line.leaves).
    
src/black/lines.py