Min aggregation does not work correctly for strings

```
input("b") .
input("a") .
output(#min(?i)) :- input(?i) .
```
Expected: "a"
Got: "b"

The first string in the program seems to be the one selected by `#min`.

No source file or test is named in the issue. Reproduce the provided program and trace the string handling for #min; done means the result is "a" rather than the first input string, with coverage for this case.

Written by the indexing model from the issue text.