old_contents: var isTag = require("domelementtype").isTag;
  
  module.exports = {
  	filter: filter,
  	find: find,
  	findOneChild: findOneChild,
  	findOne: findOne,
  	existsOne: existsOne,
  	findAll: findAll
  };
  
  function filter(test, element, recurse, limit){
  	if(!Array.isArray(element)) element = [element];
  
  	if(typeof limit !== "number" || !isFinite(limit)){
  		limit = Infinity;
  	}
  	return find(test, element, recurse !== false, limit);
  }
  
  function find(test, elems, recurse, limit){
  	var result = [], childs;
  
  	for(var i = 0, j = elems.length; i < j; i++){
  		if(test(elems[i])){
  			result.push(elems[i]);
  			if(--limit <= 0) break;
  		}
  
  		childs = elems[i].children;
  		if(recurse && childs && childs.length > 0){
  			childs = find(test, childs, recurse, limit);
  			result = result.concat(childs);
  			limit -= childs.length;
  			if(limit <= 0) break;
  		}
  	}
  
  	return result;
  }
  
  function findOneChild(test, elems){
  	for(var i = 0, l = elems.length; i < l; i++){
  		if(test(elems[i])) return elems[i];
  	}
  
  	return null;
  }
  
  function findOne(test, elems){
  	var elem = null;
  
  	for(var i = 0, l = elems.length; i < l && !elem; i++){
  		if(!isTag(elems[i])){
  			continue;
  		} else if(test(elems[i])){
  			elem = elems[i];
  		} else if(elems[i].children.length > 0){
  			elem = findOne(test, elems[i].children);
  		}
  	}
  
  	return elem;
  }
  
  function existsOne(test, elems){
  	for(var i = 0, l = elems.length; i < l; i++){
  		if(
  			isTag(elems[i]) && (
  				test(elems[i]) || (
  					elems[i].children.length > 0 &&
  					existsOne(test, elems[i].children)
  				)
  			)
  		){
  			return true;
  		}
  	}
  
  	return false;
  }
  
  function findAll(test, rootElems){
  	var result = [];
  	var stack = [rootElems];
  	while(stack.length){
  		var elems = stack.pop();
  		for(var i = 0, j = elems.length; i < j; i++){
  			if(!isTag(elems[i])) continue;
  			if(test(elems[i])) result.push(elems[i]);
  		}
  		while(j-- > 0){
  			if(elems[j].children.length > 0){
  				stack.push(elems[j].children);
  			}
  		}
  	}
  	return result;
  }
  
new_contents: var isTag = require("domelementtype").isTag;
  
  module.exports = {
  	filter: filter,
  	find: find,
  	findOneChild: findOneChild,
  	findOne: findOne,
  	existsOne: existsOne,
  	findAll: findAll
  };
  
  function filter(test, element, recurse, limit){
  	if(!Array.isArray(element)) element = [element];
  
  	if(typeof limit !== "number" || !isFinite(limit)){
  		limit = Infinity;
  	}
  	return find(test, element, recurse !== false, limit);
  }
  
  function find(test, elems, recurse, limit){
  	var result = [], childs;
  
  	for(var i = 0, j = elems.length; i < j; i++){
  		if(test(elems[i])){
  			result.push(elems[i]);
  			if(--limit <= 0) break;
  		}
  
  		childs = elems[i].children;
  		if(recurse && childs && childs.length > 0){
  			childs = find(test, childs, recurse, limit);
  			result = result.concat(childs);
  			limit -= childs.length;
  			if(limit <= 0) break;
  		}
  	}
  
  	return result;
  }
  
  function findOneChild(test, elems){
  	for(var i = 0, l = elems.length; i < l; i++){
  		if(test(elems[i])) return elems[i];
  	}
  
  	return null;
  }
  
  function findOne(test, elems){
  	var elem = null;
  
  	for(var i = 0, l = elems.length; i < l && !elem; i++){
  		if(!isTag(elems[i])){
  			continue;
  		} else if(test(elems[i])){
  			elem = elems[i];
  		} else if(elems[i].children.length > 0){
  			elem = findOne(test, elems[i].children);
  		}
  	}
  
  	return elem;
  }
  
  function existsOne(test, elems){
  	for(var i = 0, l = elems.length; i < l; i++){
  		if(
  			isTag(elems[i]) && (
  				test(elems[i]) || (
  					elems[i].children.length > 0 &&
  					existsOne(test, elems[i].children)
  				)
  			)
  		){
  			return true;
  		}
  	}
  
  	return false;
  }
  
  function findAll(test, rootElems){
  	var result = [];
  	var stack = [rootElems];
  	while(stack.length){
  		var elems = stack.pop();
  		for(var i = 0, j = elems.length; i < j; i++){
  			if(!isTag(elems[i])) continue;
  			if(test(elems[i])) result.push(elems[i]);
  		}
  		while(j-- > 0){
  			if(isTag(elems[i]) && elems[j].children.length > 0){
  				stack.push(elems[j].children);
  			}
  		}
  	}
  	return result;
  }
  
subject: Add `isTag` check before accessing children