diff --git a/Lib/tb.py b/Lib/tb.py
new file mode 100644
index 0000000..e0f2748
--- /dev/null
+++ b/Lib/tb.py
@@ -0,0 +1,220 @@
+# Print tracebacks, with a dump of local variables.
+# Also an interactive stack trace browser.
+
+import sys
+try:
+	import mac
+	os = mac
+except NameError:
+	import posix
+	os = posix
+from stat import *
+import string
+
+def br(): browser(sys.last_traceback)
+
+def tb(): printtb(sys.last_traceback)
+
+def browser(tb):
+	if not tb:
+		print 'No traceback.'
+		return
+	tblist = []
+	while tb:
+		tblist.append(tb)
+		tb = tb.tb_next
+	ptr = len(tblist)-1
+	tb = tblist[ptr]
+	while 1:
+		if tb <> tblist[ptr]:
+			tb = tblist[ptr]
+			print `ptr` + ':',
+			printtbheader(tb)
+		try:
+			line = raw_input('TB: ')
+		except KeyboardInterrupt:
+			print '\n[Interrupted]'
+			break
+		except EOFError:
+			print '\n[EOF]'
+			break
+		cmd = string.strip(line)
+		if cmd:
+			if cmd = 'quit':
+				break
+			elif cmd = 'list':
+				browserlist(tb)
+			elif cmd = 'up':
+				if ptr-1 >= 0: ptr = ptr-1
+				else: print 'Bottom of stack.'
+			elif cmd = 'down':
+				if ptr+1 < len(tblist): ptr = ptr+1
+				else: print 'Top of stack.'
+			elif cmd = 'locals':
+				printsymbols(tb.tb_frame.f_locals)
+			elif cmd = 'globals':
+				printsymbols(tb.tb_frame.f_globals)
+			elif cmd in ('?', 'help'):
+				browserhelp()
+			else:
+				browserexec(tb, cmd)
+
+def browserlist(tb):
+	filename = tb.tb_frame.f_code.co_filename
+	lineno = tb.tb_lineno
+	last = lineno
+	first = max(1, last-10)
+	for i in range(first, last+1):
+		if i = lineno: prefix = '***' + string.rjust(`i`, 4) + ':'
+		else: prefix = string.rjust(`i`, 7) + ':'
+		line = readfileline(filename, i)
+		if line[-1:] = '\n': line = line[:-1]
+		print prefix + line
+
+def browserexec(tb, cmd):
+	locals = tb.tb_frame.f_locals
+	globals = tb.tb_frame.f_globals
+	try:
+		exec(cmd+'\n', globals, locals)
+	except:
+		print '*** Exception:',
+		print sys.exc_type,
+		if sys.exc_value <> None:
+			print ':', sys.exc_value,
+		print
+		print 'Type help to get help.'
+
+def browserhelp():
+	print
+	print '    This is the traceback browser.  Commands are:'
+	print '        up      : move one level up in the call stack'
+	print '        down    : move one level down in the call stack'
+	print '        locals  : print all local variables at this level'
+	print '        globals : print all global variables at this level'
+	print '        list    : list source code around the failure'
+	print '        help    : print help (what you are reading now)'
+	print '        quit    : back to command interpreter'
+	print '    Typing any other 1-line statement will execute it'
+	print '    using the current level\'s symbol tables'
+	print
+
+def printtb(tb):
+	while tb:
+		print1tb(tb)
+		tb = tb.tb_next
+
+def print1tb(tb):
+	printtbheader(tb)
+	if tb.tb_frame.f_locals is not tb.tb_frame.f_globals:
+		printsymbols(tb.tb_frame.f_locals)
+
+def printtbheader(tb):
+	filename = tb.tb_frame.f_code.co_filename
+	lineno = tb.tb_lineno
+	info = '"' + filename + '"(' + `lineno` + ')'
+	line = readfileline(filename, lineno)
+	if line:
+		info = info + ': ' + string.strip(line)
+	print info
+
+def printsymbols(d):
+	keys = d.keys()
+	keys.sort()
+	for name in keys:
+		print '  ' + string.ljust(name, 12) + ':',
+		printobject(d[name], 4)
+		print
+
+def printobject(v, maxlevel):
+	if v = None:
+		print 'None',
+	elif type(v) in (type(0), type(0.0)):
+		print v,
+	elif type(v) = type(''):
+		if len(v) > 20:
+			print `v[:17] + '...'`,
+		else:
+			print `v`,
+	elif type(v) = type(()):
+		print '(',
+		printlist(v, maxlevel)
+		print ')',
+	elif type(v) = type([]):
+		print '[',
+		printlist(v, maxlevel)
+		print ']',
+	elif type(v) = type({}):
+		print '{',
+		printdict(v, maxlevel)
+		print '}',
+	else:
+		print v,
+
+def printlist(v, maxlevel):
+	n = len(v)
+	if n = 0: return
+	if maxlevel <= 0:
+		print '...',
+		return
+	for i in range(min(6, n)):
+		printobject(v[i], maxlevel-1)
+		if i+1 < n: print ',',
+	if n > 6: print '...',
+
+def printdict(v, maxlevel):
+	keys = v.keys()
+	n = len(keys)
+	if n = 0: return
+	if maxlevel <= 0:
+		print '...',
+		return
+	keys.sort()
+	for i in range(min(6, n)):
+		key = keys[i]
+		print `key` + ':',
+		printobject(v[key], maxlevel-1)
+		if i+1 < n: print ',',
+	if n > 6: print '...',
+
+_filecache = {}
+
+def readfileline(filename, lineno):
+	try:
+		stat = os.stat(filename)
+	except os.error, msg:
+		print 'Cannot stat', filename, '--', msg
+		return ''
+	cache_ok = 0
+	if _filecache.has_key(filename):
+		cached_stat, lines = _filecache[filename]
+		if stat[ST_SIZE] = cached_stat[ST_SIZE] and \
+				stat[ST_MTIME] = cached_stat[ST_MTIME]:
+			cache_ok = 1
+		else:
+			print 'Stale cache entry for', filename
+			del _filecache[filename]
+	if not cache_ok:
+		lines = readfilelines(filename)
+		if not lines:
+			return ''
+		_filecache[filename] = stat, lines
+	if 0 <= lineno-1 < len(lines):
+		return lines[lineno-1]
+	else:
+		print 'Line number out of range, last line is', len(lines)
+		return ''
+
+def readfilelines(filename):
+	try:
+		fp = open(filename, 'r')
+	except:
+		print 'Cannot open', filename
+		return []
+	lines = []
+	while 1:
+		line = fp.readline()
+		if not line: break
+		lines.append(line)
+	if not lines:
+		print 'Empty file', filename
+	return lines