function isMultiSliceString( prop ) {
	return (
		prop[ 0 ] === 'M' &&
		startsWith( prop, 'MultiSlice(', 0 ) &&
		prop[ prop.length-1 ] === ')'
	);
}
Tests if an indexing expression is a serialized MultiSlice object.

@private
@param {string} prop - property name
@returns {boolean} result

@example
var out = isMultiSliceString( 'MultiSlice(Slice(0,10,2),null,2,Slice(10,5,-1))' );
// returns true

@example
var out = isMultiSliceString( ':' );
// returns false