Add a debug assert.

src/simple_lottery/algorithm.rs

@@ -15,6 +15,8 @@ pub(super) fn prove(setup: &Setup, prover_set: &[Element]) -> Option<Proof> {
         }
         if prover_set.len() as u64 >= setup.proof_size {
             element_sequence.sort();
+            // Check that the elements in the sequence are unique.
+            debug_assert!(element_sequence.is_sorted_by(|a, b| a < b));
             return Some(Proof { element_sequence });
         }
     }