function next() {
		idx += 1;
		debug( 'Collection element %d: %s.', idx, JSON.stringify( collection[ idx ] ) );
		if ( predicate.length === 2 ) {
			predicate.call( opts.thisArg, collection[ idx ], clbk );
		} else if ( predicate.length === 3 ) {
			predicate.call( opts.thisArg, collection[ idx ], idx, clbk );
		} else {
			predicate.call( opts.thisArg, collection[ idx ], idx, collection, clbk ); // eslint-disable-line max-len
		}
	}
Callback to invoke a provided function for the next element in a collection.

@private