function findSetIndex(ele) {
      for (var i = 0; i < forest.length; i++) {
        var eles = forest[i];

        if (eles.has(ele)) {
          return i;
        }
      }
    }
function findSetIndex(ele) {
      for (var i = 0; i < forest.length; i++) {
        var eles = forest[i];

        if (eles.has(ele)) {
          return i;
        }
      }
    }
function findSetIndex ( ele ) { for ( var i = 0 ; i < forest . length ; i ++ ) { var eles = forest [ i ] ; if ( eles . has ( ele ) ) { return i ; } } }
assumes byGroup() creates new collections that can be safely mutated
assumes byGroup () creates new collections that can be safely mutated
Find the index of an element in the set