function flattenClbk( value, indices ) {
	if ( !isNonNegativeInteger( value ) ) {
		throw new TypeError( format( 'invalid argument. First argument must contain nonnegative integers. Indices: (%s). Value: `%s`.', indices.join( ', ' ), String( value ) ) );
	}
	return value;
}
Callback invoked upon flattening an array-like object.

@private
@param {NonNegativeInteger} value - nested array element
@param {NonNegativeIntegerArray} indices - element indices
@param {Collection<Collection>} arr - input array
@throws {TypeError} first argument must be a nonnegative integer
@returns {NonNegativeInteger} nested array element