Aggregate rule panics, if aggregate variable and distinct variable are the same

[matzemathics]

The following rule

r(#MAX(?x, ?x)) :- p(?x).

leads to a panic:

thread 'main' panicked at nemo-physical/src/util/mapping/ordered_choice.rs:37:9:
assertion failed: source.len() >= target.len()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This should probably be rejected by program analysis.