small comment change

src/circuit.rs

         if curr != 0 {
             // Gate(A_i = B_i)
             let eq_gate = Gate::new(GateType::Equal, vec![], vec![curr - 1, curr - 1 + n]);
-            // remember which gate-index this bit belongs to.
+            // remember which key-index (see below) this bit belongs to.
             indices[n - curr] = all_gates.len();
             all_gates.push(eq_gate);
         }