create a clean instance of $LAB
function create_sandbox() {
		var global_defaults = {},
			can_use_preloading = real_preloading || xhr_or_cache_preloading,
			queue = [],
			registry = {},
			instanceAPI
		;

		// global defaults
		global_defaults[_UseLocalXHR] = true;
		global_defaults[_UseCORSXHR] = false;
		global_defaults[_AlwaysPreserveOrder] = false;
		global_defaults[_AllowDuplicates] = false;
		global_defaults[_CacheBust] = false;
		/*!START_DEBUG*/global_defaults[_Debug] = true;/*!END_DEBUG*/
		global_defaults[_BasePath] = "";

		// execute a script that has been preloaded already
		function execute_preloaded_script(chain_opts,script_obj,registry_item) {
			var script;

			function preload_execute_finished() {
				if (script != null) { // make sure this only ever fires once
					script = null;
					script_executed(registry_item);
				}
			}

			if (registry[script_obj.src].finished) return;
			if (!chain_opts[_AllowDuplicates]) registry[script_obj.src].finished = true;

			script = registry_item.elem || document.createElement("script");
			if (script_obj.type) script.type = script_obj.type;
			if (script_obj.charset) script.charset = script_obj.charset;
			if (script_obj.crossOrigin) script.crossOrigin = script_obj.crossOrigin;
			create_script_load_listener(script,registry_item,"finished",preload_execute_finished);

			// script elem was real-preloaded
			if (registry_item.elem) {
				registry_item.elem = null;
			}
			// script was XHR preloaded
			else if (registry_item.text) {
				script.onload = script.onreadystatechange = null;	// script injection doesn't fire these events
				script.text = registry_item.text;
			}
			// script was cache-preloaded
			else {
				script.src = script_obj.real_src;
			}
			append_to.insertBefore(script,append_to.firstChild);

			// manually fire execution callback for injected scripts, since events don't fire
			if (registry_item.text) {
				preload_execute_finished();
			}
		}

		// process the script request setup
		function do_script(chain_opts,script_obj,chain_group,preload_this_script) {
			var registry_item,
				registry_items,
				ready_cb = function(){ script_obj.ready_cb(script_obj,function(){ execute_preloaded_script(chain_opts,script_obj,registry_item); }); },
				finished_cb = function(){ script_obj.finished_cb(script_obj,chain_group); }
			;

			script_obj.src = canonical_uri(script_obj.src,chain_opts[_BasePath]);
			script_obj.real_src = script_obj.src +
				// append cache-bust param to URL?
				(chain_opts[_CacheBust] ? ((/\?.*$/.test(script_obj.src) ? "&_" : "?_") + ~~(Math.random()*1E9) + "=") : "")
			;

			if (!registry[script_obj.src]) registry[script_obj.src] = {items:[],finished:false};
			registry_items = registry[script_obj.src].items;

			// allowing duplicates, or is this the first recorded load of this script?
			if (chain_opts[_AllowDuplicates] || registry_items.length == 0) {
				registry_item = registry_items[registry_items.length] = {
					ready:false,
					finished:false,
					ready_listeners:[ready_cb],
					finished_listeners:[finished_cb]
				};

				request_script(chain_opts,script_obj,registry_item,
					// which callback type to pass?
					(
					 	(preload_this_script) ? // depends on script-preloading
						function(){
							registry_item.ready = true;
							for (var i=0; i<registry_item.ready_listeners.length; i++) {
								registry_item.ready_listeners[i]();
							}
							registry_item.ready_listeners = [];
						} :
						function(){ script_executed(registry_item); }
					),
					// signal if script-preloading should be used or not
					preload_this_script
				);
			}
			else {
				registry_item = registry_items[0];
				if (registry_item.finished) {
					finished_cb();
				}
				else {
					registry_item.finished_listeners.push(finished_cb);
				}
			}
		}

		// creates a closure for each separate chain spawned from this $LAB instance, to keep state cleanly separated between chains
		function create_chain() {
			var chainedAPI,
				chain_opts = merge_objs(global_defaults,{}),
				chain = [],
				exec_cursor = 0,
				scripts_currently_loading = false,
				chain_is_corked = false,
				group
			;

			// called when a script has finished preloading
			function chain_script_ready(script_obj,exec_trigger) {
				if (chain_opts[_Debug]) log_msg("script preload finished: "+script_obj.real_src);
				script_obj.ready = true;
				script_obj.exec_trigger = function() {
					if (chain_opts[_Debug]) log_msg("script execute start: "+script_obj.real_src);
					exec_trigger();
				}
				advance_exec_cursor(); // will only check for 'ready' scripts to be executed
			}

			// called when a script has finished executing
			function chain_script_executed(script_obj,chain_group) {
				if (chain_opts[_Debug]) log_msg("script execution finished: "+script_obj.real_src);
				script_obj.ready = script_obj.finished = true;
				script_obj.exec_trigger = null;
				// check if chain group is all finished
				for (var i=0; i<chain_group.scripts.length; i++) {
					if (!chain_group.scripts[i].finished) return;
				}
				// chain_group is all finished if we get this far
				chain_group.finished = true;
				advance_exec_cursor();
			}

			// main driver for executing each part of the chain
			function advance_exec_cursor() {
				if (chain_is_corked) return;
				while (exec_cursor < chain.length) {
					if (is_func(chain[exec_cursor])) {
						if (chain_opts[_Debug]) log_msg("$LAB.wait() executing: "+chain[exec_cursor]);
						try { chain[exec_cursor++](); } catch (err) {
							if (chain_opts[_Debug]) log_error("$LAB.wait() error caught: ",err);
						}
						continue;
					}
					else if (!chain[exec_cursor].finished) {
						if (check_chain_group_scripts_ready(chain[exec_cursor])) continue;
						break;
					}
					exec_cursor++;
				}
				// we've reached the end of the chain (so far)
				if (exec_cursor == chain.length) {
					scripts_currently_loading = false;
					group = false;
				}
			}

			// setup next chain script group
			function init_script_chain_group() {
				if (!group || !group.scripts) {
					chain.push(group = {scripts:[],finished:true});
				}
			}

			// API for $LAB chains
			chainedAPI = {
				// start loading one or more scripts
				script:function(){
					for (var i=0; i<arguments.length; i++) {
						(function(script_obj,script_list){
							var splice_args;

							if (!is_array(script_obj)) {
								script_list = [script_obj];
							}
							for (var j=0; j<script_list.length; j++) {
								init_script_chain_group();
								script_obj = script_list[j];

								if (is_func(script_obj)) script_obj = script_obj();
								if (!script_obj) continue;
								if (is_array(script_obj)) {
									// set up an array of arguments to pass to splice()
									splice_args = [].slice.call(script_obj); // first include the actual array elements we want to splice in
									splice_args.unshift(j,1); // next, put the `index` and `howMany` parameters onto the beginning of the splice-arguments array
									[].splice.apply(script_list,splice_args); // use the splice-arguments array as arguments for splice()
									j--; // adjust `j` to account for the loop's subsequent `j++`, so that the next loop iteration uses the same `j` index value
									continue;
								}
								if (typeof script_obj == "string") script_obj = {src:script_obj};
								script_obj = merge_objs(script_obj,{
									ready:false,
									ready_cb:chain_script_ready,
									finished:false,
									finished_cb:chain_script_executed
								});
								group.finished = false;
								group.scripts.push(script_obj);
								do_script(chain_opts,script_obj,group,(
									(can_use_preloading && scripts_currently_loading) || chain_is_corked
								));
								scripts_currently_loading = true;

								if (chain_opts[_AlwaysPreserveOrder]) chainedAPI.wait();
							}
						})(arguments[i],arguments[i]);
					}
					return chainedAPI;
				},
				// force LABjs to pause in execution at this point in the chain, until the execution thus far finishes, before proceeding
				wait:function(){
					if (arguments.length > 0) {
						for (var i=0; i<arguments.length; i++) {
							chain.push(arguments[i]);
						}
						group = chain[chain.length-1];
					}
					else group = false;

					advance_exec_cursor();

					return chainedAPI;
				},
				cork:function(){
					if (chain_opts[_Debug]) log_msg("$LAB.cork()");
					chain_is_corked = true;
					return chainedAPI;
				},
				uncork:function(){
					if (chain_opts[_Debug]) log_msg("$LAB.uncork()");
					if (chain_is_corked) {
						chain_is_corked = false;
						advance_exec_cursor();
					}
					return chainedAPI;
				}
			};

			// the first chain link API (includes `setOptions` only this first time)
			return {
				script:chainedAPI.script,
				wait:chainedAPI.wait,
				cork:chainedAPI.cork,
				uncork:chainedAPI.uncork,
				setOptions:function(opts){
					merge_objs(opts,chain_opts);
					return chainedAPI;
				}
			};
		}

		// API for each initial $LAB instance (before chaining starts)
		instanceAPI = {
			// main API functions
			setGlobalDefaults:function(opts){
				merge_objs(opts,global_defaults);
				return instanceAPI;
			},
			setOptions:function(){
				return create_chain().setOptions.apply(null,arguments);
			},
			script:function(){
				return create_chain().script.apply(null,arguments);
			},
			wait:function(){
				return create_chain().wait.apply(null,arguments);
			},
			cork:function(){
				return create_chain().cork.apply(null,arguments);
			},

			// built-in queuing for $LAB `script()` and `wait()` calls
			// useful for building up a chain programmatically across various script locations, and simulating
			// execution of the chain
			queueScript:function(){
				queue[queue.length] = {type:"script", args:[].slice.call(arguments)};
				return instanceAPI;
			},
			queueWait:function(){
				queue[queue.length] = {type:"wait", args:[].slice.call(arguments)};
				return instanceAPI;
			},
			runQueue:function(){
				var $L = instanceAPI, len=queue.length, i=len, val;
				for (;--i>=0;) {
					val = queue.shift();
					$L = $L[val.type].apply(null,val.args);
				}
				return $L;
			},

			// rollback `[global].$LAB` to what it was before this file was loaded, the return this current instance of $LAB
			noConflict:function(){
				global.$LAB = _$LAB;
				return instanceAPI;
			},

			// create another clean instance of $LAB
			sandbox:function(){
				return create_sandbox();
			}
		};

		return instanceAPI;
	}