code: function(idx) {
  	let requiredSteps = [idx];
  	
  	let step = toR.filter(function (i) {
  	    return i.idx == idx;
  	})[0];
  
  
  	if (!step.req)
  	    return requiredSteps;
  
  	step.req.forEach(function (i) {
  	    requiredSteps = requiredSteps.concat(findRequiredSteps(i));
  	});
  
  	return requiredSteps;
      }
docs: /*	neg.idx = pc++;
  kb.push(neg);
  toR.push({label: "assume for a contradiction",
  tree: kb.peek(),
  idx: kb.peek().idx});