# Back Operator

The `>back` operator returns to the state that most recently transitioned into the
current state. In state diagrams it is indicated by dashed edges.

In this example, when the `>back` operator is triggered by `Port3_High` in the
state `WaitForExit`, the state machine returns to the state that previously
transitioned into `WaitForExit` - either `FlashPort1` or `FlashPort2`.

![image](state_machines/examples/back_operator__light.svg)![image](state_machines/examples/back_operator__dark.svg)

### Python

```python
from bpod_core.fsm import StateMachine

fsm = StateMachine()

fsm.add_state(
    name='WaitForChoice',
    transitions={'Port1_High': 'FlashPort1', 'Port2_High': 'FlashPort2'},
)
fsm.add_state(
    name='FlashPort1',
    timer=0.5,
    transitions={'Tup': 'WaitForExit'},
    actions={'PWM1': 255},
)
fsm.add_state(
    name='FlashPort2',
    timer=0.5,
    transitions={'Tup': 'WaitForExit'},
    actions={'PWM2': 255},
)
fsm.add_state(
    name='WaitForExit',
    transitions={
        'Port1_High': '>exit',
        'Port2_High': '>exit',
        'Port3_High': '>back',
    },
)
```

### JSON

```json
{
  "states": {
    "WaitForChoice": {
      "transitions": {
        "Port1_High": "FlashPort1",
        "Port2_High": "FlashPort2"
      }
    },
    "FlashPort1": {
      "timer": 0.5,
      "transitions": {
        "Tup": "WaitForExit"
      },
      "actions": {
        "PWM1": 255
      }
    },
    "FlashPort2": {
      "timer": 0.5,
      "transitions": {
        "Tup": "WaitForExit"
      },
      "actions": {
        "PWM2": 255
      }
    },
    "WaitForExit": {
      "transitions": {
        "Port1_High": ">exit",
        "Port2_High": ">exit",
        "Port3_High": ">back"
      }
    }
  }
}
```

### YAML

```yaml
states:
  WaitForChoice:
    transitions:
      Port1_High: FlashPort1
      Port2_High: FlashPort2
  FlashPort1:
    timer: 0.5
    transitions:
      Tup: WaitForExit
    actions:
      PWM1: 255
  FlashPort2:
    timer: 0.5
    transitions:
      Tup: WaitForExit
    actions:
      PWM2: 255
  WaitForExit:
    transitions:
      Port1_High: '>exit'
      Port2_High: '>exit'
      Port3_High: '>back'
```