<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 system consists of two links connected linearly to form a chain, with one end of
the chain fixed. The joint between the two links is actuated. The goal is to apply
torques on the actuated joint to swing the free end of the linear chain above a
given height while starting from the initial state of hanging downwards.

As seen in the **Gif**: two blue links connected by two green joints. The joint in
between the two links is actuated. The goal is to swing the free end of the outer-link
to reach the target height (black horizontal line above system) by applying torque on
the actuator.

## Action Space

The action is discrete, deterministic, and represents the torque applied on the actuated
joint between the two links.

| Num | Action                                | Unit         |
|-----|---------------------------------------|--------------|
| 0   | apply -1 torque to the actuated joint | torque (N m) |
| 1   | apply 0 torque to the actuated joint  | torque (N m) |
| 2   | apply 1 torque to the actuated joint  | torque (N m) |

## Observation Space

The observation is a `ndarray` with shape `(6,)` that provides information about the
two rotational joint angles as well as their angular velocities:

| Num | Observation                  | Min                 | Max               |
|-----|------------------------------|---------------------|-------------------|
| 0   | Cosine of `theta1`           | -1                  | 1                 |
| 1   | Sine of `theta1`             | -1                  | 1                 |
| 2   | Cosine of `theta2`           | -1                  | 1                 |
| 3   | Sine of `theta2`             | -1                  | 1                 |
| 4   | Angular velocity of `theta1` | ~ -12.567 (-4 * pi) | ~ 12.567 (4 * pi) |
| 5   | Angular velocity of `theta2` | ~ -28.274 (-9 * pi) | ~ 28.274 (9 * pi) |

where
- `theta1` is the angle of the first joint, where an angle of 0 indicates the first link is pointing directly
downwards.
- `theta2` is ***relative to the angle of the first link.***
    An angle of 0 corresponds to having the same angle between the two links.

The angular velocities of `theta1` and `theta2` are bounded at ±4π, and ±9π rad/s respectively.
A state of `[1, 0, 1, 0, ..., ...]` indicates that both links are pointing downwards.

## Rewards

The goal is to have the free end reach a designated target height in as few steps as possible,
and as such all steps that do not reach the goal incur a reward of -1.
Achieving the target height results in termination with a reward of 0. The reward threshold is -100.

## Starting State

Each parameter in the underlying state (`theta1`, `theta2`, and the two angular velocities) is initialized
uniformly between -0.1 and 0.1. This means both links are pointing downwards with some initial stochasticity.

## Episode End

The episode ends if one of the following occurs:
1. Termination: The free end reaches the target height, which is constructed as:
`-cos(theta1) - cos(theta2 + theta1) > 1.0`
2. Truncation: Episode length is greater than 500 (200 for v0)

## 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>