verify list of indices are a valid permutation in circom zk circuit

verify.circom

pragma circom 2.1.5;

include "circomlib/circuits/comparators.circom";

/*
  Verifies a permutation is valid and includes all indices once and only once.
  Examples:
  [ 0 1 2 3 ] == OK
  [ 1 2 0 3 ] == OK
  [ 0 2 0 3 ] == NOT OK
  [ 1 1 2 3 ] == NOT OK
*/

template Verifier(N) {
    signal input indices[N];
    signal output out;

    // Check all values are < N
    component lt[N];
    for (var i = 0; i < N; i++) {
      var num = indices[i];

      // 1. Ensure num is less than N
      // numMaxBits = 16, max number of elements in the array must be < 2^16    
      lt[i] = LessThan(16);
      lt[i].in[0] <== num;
      lt[i].in[1] <== N;
      lt[i].out === 1; // assert
    }

    // Check that each element is visited once
    var visited[N];
    for (var i = 0; i < N; i++) {
      visited[i] = 0;
    }
    for (var i = 0; i < N; i++) {
      visited[indices[i]] = 1;
    }
    var sum = 0;
    for (var i = 0; i < N; i++) {
      sum += visited[i];
    }

    // ensure that the sum adds up to the N value
    component eq = IsEqual();
    eq.in[0] <-- sum; // note: no constraint here; is this a problem?
    eq.in[1] <== N;
    eq.out === 1;

    out <== eq.out;
}

// Operates on a fixed array of N size
component main = Verifier(4);