Transforms a list of edges into a list of the nodes those edges connect.
    Returns a list of nodes, or an empty list if given an empty list.

def get_vertices_from_edge_list(graph, edge_list):
    """Transforms a list of edges into a list of the nodes those edges connect.
    Returns a list of nodes, or an empty list if given an empty list.
    """
    node_set = set()
    for edge_id in edge_list:
        edge = graph.get_edge(edge_id)
        a, b = edge['vertices']
        node_set.add(a)
        node_set.add(b)

    return list(node_set)