Solve the programming task below in a Python markdown code block.
# Description

Write a function that accepts the current position of a knight in a chess board, it returns the possible positions that it will end up after 1 move. The resulted should be sorted. 

## Example

"a1" -> ["b3", "c2"]
Also feel free to reuse/extend the following starter code:
```python
def possible_positions(pos):
```