def build_tree(items):
    """
    This function takes a Python list of items and turns it into a binary tree
    of the items, suitable for casting to an s-expression.
    """
    size = len(items)
    if size == 0:
        return []
    if size == 1:
        return items[0]
    half_size = size >> 1
    left = build_tree(items[:half_size])
    right = build_tree(items[half_size:])
    return (left, right)