function parseMultiSlice( raw, str ) {
	var s = str2multislice( str );
	if ( s === null ) {
		throw new Error( format( 'invalid operation. Unsupported slice operation. Value: `%s`.', raw ) );
	}
	return s;
}
Parses a serialized MultiSlice object.

@private
@param {string} raw - original unprocessed input string
@param {string} str - serialized MultiSlice object
@throws {Error} invalid slice operation
@returns {MultiSlice} MultiSlice object

@example
var s = parseMultiSlice( '  MultiSlice(Slice(0,10,2),null,2,Slice(10,5,-1))  ', 'MultiSlice(Slice(0,10,2),null,2,Slice(10,5,-1))' );
// returns <MultiSlice>