function groupBy( collection, opts, indicator ) {
	var thisArg;
	var out;
	var len;
	var g;
	var v;
	var i;

	thisArg = opts.thisArg;
	len = collection.length;

	out = {};
	for ( i = 0; i < len; i++ ) {
		v = collection[ i ];
		g = indicator.call( thisArg, v, i );
		if ( hasOwnProp( out, g ) ) {
			out[ g ].push( [ i, v ] );
		} else {
			out[ g ] = [ [ i, v ] ];
		}
	}
	return out;
}
Groups values according to an indicator function and outputs results as index-element values.

## Notes

-   We need to cache the collection value to prevent the edge case where, during the invocation of the indicator function, the element at index `i` is swapped for some other value. For some, that might be a feature; here, we take the stance that one should be less clever.
-   Checking for an "own" property is necessary to guard against the edge case where an indicator function returns a group identifier which matches a method or property on the `Object` prototype.

@private
@param {Collection} collection - collection to group
@param {Options} opts - function options
@param {*} [opts.thisArg] - execution context
@param {Function} indicator - indicator function specifying which group an element in the input collection belongs to
@returns {Object} group results

@example
function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupBy( arr, {}, indicator );
// returns { 'b': [ [ 0, 'beep' ], [ 1, 'boop' ], [ 3, 'bar' ] ], 'f': [ [ 2, 'foo' ] ] }