<system>
You are an experienced Python developer. You will be provided with an incorrect Python program. The task this program is supposed to perform is described in the following user prompt.
Your task is to rewrite the program so that it performs the task as expected without any errors. You will be rewarded based on the number of test cases your code passes.
</system>

<user>
## Description
The environment aims to
increase the number of independent state and control variables as compared to
the classic control environments. The hopper is a two-dimensional
one-legged figure that consist of four main body parts - the torso at the
top, the thigh in the middle, the leg in the bottom, and a single foot on
which the entire body rests. The goal is to make hops that move in the
forward (right) direction by applying torques on the three hinges
connecting the four body parts.

## Action Space
The action space is a `Box(-1, 1, (3,), float32)`. An action represents the torques applied at the hinge joints.

| Num | Action                             | Control Min | Control Max | Name (in corresponding XML file) | Joint | Unit         |
|-----|------------------------------------|-------------|-------------|----------------------------------|-------|--------------|
| 0   | Torque applied on the thigh rotor  | -1          | 1           | thigh_joint                      | hinge | torque (N m) |
| 1   | Torque applied on the leg rotor    | -1          | 1           | leg_joint                        | hinge | torque (N m) |
| 2   | Torque applied on the foot rotor   | -1          | 1           | foot_joint                       | hinge | torque (N m) |

## Observation Space
Observations consist of positional values of different body parts of the
hopper, followed by the velocities of those individual parts
(their derivatives) with all the positions ordered before all the velocities.

By default, observations do not include the x-coordinate of the hopper. It may
be included by passing `exclude_current_positions_from_observation=False` during construction.
In that case, the observation space will be `Box(-Inf, Inf, (12,), float64)` where the first observation
represents the x-coordinate of the hopper.
Regardless of whether `exclude_current_positions_from_observation` was set to true or false, the x-coordinate
will be returned in `info` with key `"x_position"`.

However, by default, the observation is a `Box(-Inf, Inf, (11,), float64)` where the elements
correspond to the following:

| Num | Observation                                        | Min  | Max | Name (in corresponding XML file) | Joint | Unit                     |
| --- | -------------------------------------------------- | ---- | --- | -------------------------------- | ----- | ------------------------ |
| 0   | z-coordinate of the torso (height of hopper)       | -Inf | Inf | rootz                            | slide | position (m)             |
| 1   | angle of the torso                                 | -Inf | Inf | rooty                            | hinge | angle (rad)              |
| 2   | angle of the thigh joint                           | -Inf | Inf | thigh_joint                      | hinge | angle (rad)              |
| 3   | angle of the leg joint                             | -Inf | Inf | leg_joint                        | hinge | angle (rad)              |
| 4   | angle of the foot joint                            | -Inf | Inf | foot_joint                       | hinge | angle (rad)              |
| 5   | velocity of the x-coordinate of the torso          | -Inf | Inf | rootx                          | slide | velocity (m/s)           |
| 6   | velocity of the z-coordinate (height) of the torso | -Inf | Inf | rootz                          | slide | velocity (m/s)           |
| 7   | angular velocity of the angle of the torso         | -Inf | Inf | rooty                          | hinge | angular velocity (rad/s) |
| 8   | angular velocity of the thigh hinge                | -Inf | Inf | thigh_joint                      | hinge | angular velocity (rad/s) |
| 9   | angular velocity of the leg hinge                  | -Inf | Inf | leg_joint                        | hinge | angular velocity (rad/s) |
| 10  | angular velocity of the foot hinge                 | -Inf | Inf | foot_joint                       | hinge | angular velocity (rad/s) |
| excluded | x-coordinate of the torso                     | -Inf | Inf | rootx                            | slide | position (m)             |


## Rewards
The reward consists of three parts:
- *healthy_reward*: Every timestep that the hopper is healthy (see definition in section "Episode Termination"), it gets a reward of fixed value `healthy_reward`.
- *forward_reward*: A reward of hopping forward which is measured
as *`forward_reward_weight` * (x-coordinate before action - x-coordinate after action)/dt*. *dt* is
the time between actions and is dependent on the frame_skip parameter
(fixed to 4), where the frametime is 0.002 - making the
default *dt = 4 * 0.002 = 0.008*. This reward would be positive if the hopper
hops forward (positive x direction).
- *ctrl_cost*: A cost for penalising the hopper if it takes
actions that are too large. It is measured as *`ctrl_cost_weight` *
sum(action<sup>2</sup>)* where *`ctrl_cost_weight`* is a parameter set for the
control and has a default value of 0.001

The total reward returned is ***reward*** *=* *healthy_reward + forward_reward - ctrl_cost* and `info` will also contain the individual reward terms

## Starting State
All observations start in state
(0.0, 1.25, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0) with a uniform noise
 in the range of [-`reset_noise_scale`, `reset_noise_scale`] added to the values for stochasticity.

## Episode End
The hopper is said to be unhealthy if any of the following happens:

1. An element of `observation[1:]` (if  `exclude_current_positions_from_observation=True`, else `observation[2:]`) is no longer contained in the closed interval specified by the argument `healthy_state_range`
2. The height of the hopper (`observation[0]` if  `exclude_current_positions_from_observation=True`, else `observation[1]`) is no longer contained in the closed interval specified by the argument `healthy_z_range` (usually meaning that it has fallen)
3. The angle (`observation[1]` if  `exclude_current_positions_from_observation=True`, else `observation[2]`) is no longer contained in the closed interval specified by the argument `healthy_angle_range`

If `terminate_when_unhealthy=True` is passed during construction (which is the default),
the episode ends when any of the following happens:

1. Truncation: The episode duration reaches a 1000 timesteps
2. Termination: The hopper is unhealthy

If `terminate_when_unhealthy=False` is passed, the episode is ended only when 1000 timesteps are exceeded.

## Class Definition
The class should be called "Environment". It should have at least:

- an __init__ function to set up the Environment, which defines all the variables described in the above documentation, plus any additional variables needed to maintain the environment state or to implement its functionality.
- a set_state function to set a custom value for the environment and its internal representation (you can assume that when "set_state" is used, the task is not done and internal variables should be set as a consequence). set_state takes a single argument as input: a state observation from the observation space defined above.
- a step function to predict a step in the environment. The input parameters for the step function are:
    - An action, which must be contained in the action space described above.
  
    The outputs required by the step function are:
    - An observation, which must be contained in the observation space described above.
    - The reward for taking the action, as described in the reward definition above.
    - A boolean variable indicating if the episode is done.

## Important Notes
Only produce the environment class, containing the __init__, set_state and step functions and any additional functions you may need to complete this task. Do not write an example of how to use the class or anything else.
Be careful about edge cases.
Make sure to write all the required functions and that they have the exact names as specified in the task description. Missing or incorrectly named functions will not pass the tests and will result in a score of 0.
It is of VITAL importance that you do not leave undefined any function, but implement each of them completely.
Output the FULL corrected program in its entirety and NOTHING ELSE.

First, write an explanation of the error and point out the part of the code responsible for the error and why its logic is erroneous.
Second, suggest how you would fix the error, reasoning about the problem.
Finally fix the program in its entirety following the suggestion. The expected output is in the format:

## Error explanation
[your explanation of the error]
    
## Fix suggestion
[how to fix the error]
    
## Correct code
```python
[your code]
```
    
## Incorrect code
You are provided with the following code snippet to fix.
```python
{CODE}
```
    
{ERROR}

</user>

<assistant>
## Error explanation
</assistant>