Returns the default coin name.

The default coin name depends on the graph on which the quantum walk occurs. In general, the default coin is 'grover'. For the hiperwalk.Line and hiperwalk.Cycle, the default coin is 'hadamard'.



Although the default coin depends on the graph, the coin is not an attribute of the graph. That is, it is part of graph theory. In addition, it does not make sense to have a default coin in continuous-time quantum walks. Hence, the method is implemented in the Coined class instead of each graph class.

Following duck typing, we do not verify if the graph is an instance of the line or cycle. Instead, we verify if the maximum degree on the graph is at most 2.