def generate_pow(self, node_id, btime=None):
    ' stakes a block using generate on nodes[node_id]'
    assert_greater_than(len(self.nodes), node_id)
    self.nodes[node_id].generate(1)
    if (btime is not None):
        btime += 60
        set_node_times(self.nodes, btime)
    return btime
stakes a block using generate on nodes[node_id]
def generate_pow(self, node_id, btime=None):
    ' '
    assert_greater_than(len(self.nodes), node_id)
    self.nodes[node_id].generate(1)
    if (btime is not None):
        btime += 60
        set_node_times(self.nodes, btime)
    return btime