vertices = ["gate", "oak", "pond"]
index = {vertex: position for position, vertex in enumerate(vertices)}
matrix = [[None for _ in vertices] for _ in vertices]
matrix[index["gate"]][index["oak"]] = 0
matrix[index["gate"]][index["pond"]] = 4
print(matrix[0])
print(matrix[1][0] is None)
print(matrix[0][1] == 0)