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// create horizontal guide on masterspread
// see http://jongware.mit.edu/idcs6js/pc_Guide.html
// or http://yearbookmachine.github.io/esdocs/#/InDesign/Guide
(function(thisObj){

  var doc = app.activeDocument;
  var spread = doc.masterSpreads[0];
  spread.guides.add({
    orientation:HorizontalOrVertical.HORIZONTAL,
    location:"800px"
    });

  })(this);