from typing import Iterable
from typing import Tuple
from typing import Dict
from typing import List


def adjacency_list(edges: Iterable[Tuple[str, str]]) -> Dict[str, List[str]]:
    """
    Convert a sequence of undirected edge pairs into an adjacency "list".
    """
    adj = {}

    for e1, e2 in edges:
        if e1 not in adj:
            adj[e1] = []
        if e2 not in adj:
            adj[e2] = []

        adj[e1].append(e2)
        adj[e2].append(e1)

    return adj