How can I add an open triangle arrow-end to a line?

How can I add an open triangle arrow-end to a line in Raphael JS?

The closest to it is block-wide-long, but I would like to make an arrow-end empty inside.

var path_1 = paper.path('M10 50 L250 50');
path_1.attr({stroke:'#FF0000', 'stroke-width': 6 ,'arrow-end': 'block-wide-long'});

JSFiddle