function iterAdd() {
	var iterators;
	var types;
	var niter;
	var iter;
	var FLG;
	var i;

	niter = arguments.length;
	if ( niter < 2 ) {
		throw new Error( 'insufficient arguments. Must provide two or more iterators.' );
	}
	iterators = [];
	types = [];
	for ( i = 0; i < niter; i++ ) {
		iterators.push( arguments[ i ] );
		if ( isIteratorLike( arguments[ i ] ) ) {
			types.push( 1 );
		} else if ( isNumber( arguments[ i ] ) ) {
			types.push( 0 );
		} else {
			throw new TypeError( format( 'invalid argument. Must provide an iterator protocol-compliant object or a number. Argument: `%u`. Value: `%s`.', i, arguments[ i ] ) );
		}
	}
	// Create an iterator protocol-compliant object:
	iter = {};
	setReadOnly( iter, 'next', next );
	setReadOnly( iter, 'return', end );

	// If an environment supports `Symbol.iterator` and all provided iterators are iterable, make the iterator iterable:
	if ( iteratorSymbol ) {
		for ( i = 0; i < niter; i++ ) {
			if ( types[ i ] && !isFunction( iterators[ i ][ iteratorSymbol ] ) ) { // eslint-disable-line max-len
				FLG = true;
				break;
			}
		}
		if ( !FLG ) {
			setReadOnly( iter, iteratorSymbol, factory );
		}
	}
	FLG = false;
	i = 0;
	return iter;

	/**
	* Returns an iterator protocol-compliant object containing the next iterated value.
	*
	* @private
	* @returns {Object} iterator protocol-compliant object
	*/
	function next() {
		var s;
		var v;
		var i;
		if ( FLG ) {
			return {
				'done': true
			};
		}
		s = 0.0;
		for ( i = 0; i < niter; i++ ) {
			if ( types[ i ] ) {
				v = iterators[ i ].next();
				if ( v.done ) {
					FLG = true;
					return v;
				}
				if ( typeof v.value === 'number' ) {
					s += v.value;
				} else {
					s = NaN;
				}
			} else {
				s += iterators[ i ];
			}
		}
		return {
			'value': s,
			'done': false
		};
	}

	/**
	* Finishes an iterator.
	*
	* @private
	* @param {*} [value] - value to return
	* @returns {Object} iterator protocol-compliant object
	*/
	function end( value ) {
		FLG = true;
		if ( arguments.length ) {
			return {
				'value': value,
				'done': true
			};
		}
		return {
			'done': true
		};
	}

	/**
	* Returns a new iterator.
	*
	* @private
	* @returns {Iterator} iterator
	*/
	function factory() {
		var args;
		var i;

		args = [];
		for ( i = 0; i < niter; i++ ) {
			if ( types[ i ] ) {
				args.push( iterators[ i ][ iteratorSymbol ]() );
			} else {
				args.push( iterators[ i ] );
			}
		}
		return iterAdd.apply( null, args );
	}
}
Returns an iterator which performs element-wise addition of two or more iterators.

## Notes

-   If provided a numeric value as an iterator argument, the value is broadcast as an **infinite** iterator which **always** returns the provided value.
-   If an iterated value is non-numeric (including `NaN`), the returned iterator returns `NaN`. If non-numeric iterated values are possible, you are advised to provide an iterator which type checks and handles non-numeric values accordingly.
-   The length of the returned iterator is equal to the length of the shortest provided iterator. In other words, the returned iterator ends once **one** of the provided iterators ends.
-   If an environment supports `Symbol.iterator` and all provided iterators are iterable, the returned iterator is iterable.

@param {Iterator} iter0 - first input iterator
@param {...(Iterator|number)} iterator - subsequent iterators
@throws {Error} must provide two or more iterators
@throws {TypeError} must provide iterator protocol-compliant objects
@returns {Iterator} iterator

@example
var array2iterator = require( '@stdlib/array/to-iterator' );

var it1 = array2iterator( [ 1.0, 2.0 ] );
var it2 = array2iterator( [ 3.0, 4.0 ] );

var iter = iterAdd( it1, it2 );

var v = iter.next().value;
// returns 4.0

v = iter.next().value;
// returns 6.0

var bool = iter.next().done;
// returns true