function setDifference( a, b ) {
	var out;
	var i;
	out = [];
	for ( i = 0; i < a.length; i++ ) {
		if ( indexOf( b, a[ i ] ) === -1 ) {
			out.push( a[ i ] );
		}
	}
	return out;
}
Returns the set difference (i.e., the elements of `A` which are not in `B`).

@private
@param {StringArray} a - set `A`
@param {StringArray} b - set `B`
@returns {(StringArray|EmptyArray)} set difference