human
I wrote the below code

    if (start not in G):
        raise nx.NetworkXError('start    is    not    in    G')
    idom = {start: start}
    order = list(nx.dfs_postorder_nodes(G, start))
    dfn = {u: i for (i, u) in enumerate(order)}
    order.pop()
    order.reverse()
    def intersect(u, v):
        while (u != v):
            while (dfn[u] < dfn[v]):
                u = idom[u]
            while (dfn[u] > dfn[v]):
                v = idom[v]
        return u
    changed = True
    while changed:
        changed = False
        for u in order:
            new_idom = reduce(intersect, (v for v in G.pred[u] if (v in idom)))
            if ((u not in idom) or (idom[u] != new_idom)):
                idom[u] = new_idom
                changed = True
    return idom

What's a good function header?

Here's a good function header?
assistant
#Here's a good function header.

@not_implemented_for('undirected')
def immediate_dominators(G, start):