def validate_graph(graph, weighted=False, nonnegative=False):
    if not isinstance(graph, dict):
        raise ValueError("Expected an adjacency dictionary")
    if any(not isinstance(vertex, str) for vertex in graph):
        raise ValueError("String vertex labels required")
    for vertex, neighbors in graph.items():
        for edge in neighbors:
            if weighted:
                target, weight = edge
                if type(weight) is not int or (nonnegative and weight < 0):
                    raise ValueError("Invalid integer edge weight")
            else:
                target = edge
            if target not in graph:
                raise ValueError("Unknown target vertex")

from collections import deque

def topological_order(graph):
    validate_graph(graph)
    incoming = {vertex: 0 for vertex in graph}
    for neighbors in graph.values():
        for target in neighbors:
            incoming[target] += 1
    pending = deque(vertex for vertex in graph if incoming[vertex] == 0)
    result = []
    while pending:
        vertex = pending.popleft()
        result.append(vertex)
        for target in graph[vertex]:
            incoming[target] -= 1
            if incoming[target] == 0:
                pending.append(target)
    if len(result) != len(graph):
        raise ValueError("Directed cycle prevents an ordering")
    return result

graph = {"collect": ["clean"], "clean": ["report"], "report": [], "label": []}
print(topological_order(graph))
try:
    topological_order({"a": ["b"], "b": ["a"]})
except ValueError as error:
    print(error)