A function is idempotent when its output fed back as input returns the same result. Here's a Python example:
```python
def absolute_value(x):
    """
    # Let's try a negative input [Test 1]
    >>> absolute_value(-1)
    1
    # Feed output as input [Test 2]
    >>> absolute_value(1)
    1
    # In one line of code [Test 2 wrapping Test 1]
    >>> absolute_value(absolute_value(-1))
    1
    """
    return x if x > 0 else -1 * x
```
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