from __future__ import annotations
from dataclasses import dataclass, field
from typing import Any

@dataclass
class Node:
    id: str
    payload: dict[str, Any]
    edges: set[str] = field(default_factory=set)

class HolographicDAG:
    def __init__(self) -> None:
        self.nodes: dict[str, Node] = {}

    def add(self, node_id: str, payload: dict[str, Any]) -> Node:
        node = self.nodes.setdefault(node_id, Node(node_id, payload))
        return node

    def link(self, source: str, target: str) -> None:
        if source not in self.nodes or target not in self.nodes:
            raise KeyError("Both nodes must exist before linking")
        self.nodes[source].edges.add(target)

    def neighborhood(self, node_id: str) -> list[Node]:
        node = self.nodes[node_id]
        return [self.nodes[e] for e in node.edges]