diff --git a/lib/debugger.rb b/lib/debugger.rb
index 1ef329f..4bcfeba 100644
--- a/lib/debugger.rb
+++ b/lib/debugger.rb
@@ -2,7 +2,7 @@
 require "opal"
 require "opal-parser"
 
-
+require "logger"
 require 'browser'
 require 'browser/http'
 require 'native'
diff --git a/lib/logger.rb b/lib/logger.rb
new file mode 100644
index 0000000..c440796
--- /dev/null
+++ b/lib/logger.rb
@@ -0,0 +1,773 @@
+# Copied logger from logger gem.
+# gemoved monitor for opal
+
+# == Description
+#
+# The Logger class provides a simple but sophisticated logging utility that
+# you can use to output messages.
+#
+# The messages have associated
+# levels, such as +INFO+ or +ERROR+ that indicate their importance.
+# You can then give the Logger a level, and only messages at that
+# level of higher will be printed.
+#
+# The levels are:
+#
+# +FATAL+:: an unhandleable error that results in a program crash
+# +ERROR+:: a handleable error condition
+# +WARN+::  a warning
+# +INFO+::  generic (useful) information about system operation
+# +DEBUG+:: low-level information for developers
+#
+# For instance, in a production system, you may have your Logger set to
+# +INFO+ or even +WARN+
+# When you are developing the system, however, you probably
+# want to know about the program's internal state, and would set the Logger to
+# +DEBUG+.
+#
+# *Note*: Logger does not escape or sanitize any messages passed to it.
+# Developers should be aware of when potentially malicious data (user-input)
+# is passed to Logger, and manually escape the untrusted data:
+#
+#   logger.info("User-input: #{input.dump}")
+#   logger.info("User-input: %p" % input)
+#
+# You can use #formatter= for escaping all data.
+#
+#   original_formatter = Logger::Formatter.new
+#   logger.formatter = proc { |severity, datetime, progname, msg|
+#     original_formatter.call(severity, datetime, progname, msg.dump)
+#   }
+#   logger.info(input)
+#
+# === Example
+#
+# This creates a logger to the standard output stream, with a level of +WARN+
+#
+#   log = Logger.new(STDOUT)
+#   log.level = Logger::WARN
+#
+#   log.debug("Created logger")
+#   log.info("Program started")
+#   log.warn("Nothing to do!")
+#
+#   begin
+#     File.each_line(path) do |line|
+#       unless line =~ /^(\w+) = (.*)$/
+#         log.error("Line in wrong format: #{line}")
+#       end
+#     end
+#   rescue => err
+#     log.fatal("Caught exception; exiting")
+#     log.fatal(err)
+#   end
+#
+# Because the Logger's level is set to +WARN+, only the warning, error, and
+# fatal messages are recorded.  The debug and info messages are silently
+# discarded.
+#
+# === Features
+#
+# There are several interesting features that Logger provides, like
+# auto-rolling of log files, setting the format of log messages, and
+# specifying a program name in conjunction with the message.  The next section
+# shows you how to achieve these things.
+#
+#
+# == HOWTOs
+#
+# === How to create a logger
+#
+# The options below give you various choices, in more or less increasing
+# complexity.
+#
+# 1. Create a logger which logs messages to STDERR/STDOUT.
+#
+#      logger = Logger.new(STDERR)
+#      logger = Logger.new(STDOUT)
+#
+# 2. Create a logger for the file which has the specified name.
+#
+#      logger = Logger.new('logfile.log')
+#
+# 3. Create a logger for the specified file.
+#
+#      file = File.open('foo.log', File::WRONLY | File::APPEND)
+#      # To create new (and to remove old) logfile, add File::CREAT like;
+#      #   file = open('foo.log', File::WRONLY | File::APPEND | File::CREAT)
+#      logger = Logger.new(file)
+#
+# 4. Create a logger which ages logfile once it reaches a certain size.  Leave
+#    10 "old log files" and each file is about 1,024,000 bytes.
+#
+#      logger = Logger.new('foo.log', 10, 1024000)
+#
+# 5. Create a logger which ages logfile daily/weekly/monthly.
+#
+#      logger = Logger.new('foo.log', 'daily')
+#      logger = Logger.new('foo.log', 'weekly')
+#      logger = Logger.new('foo.log', 'monthly')
+#
+# === How to log a message
+#
+# Notice the different methods (+fatal+, +error+, +info+) being used to log
+# messages of various levels?  Other methods in this family are +warn+ and
+# +debug+.  +add+ is used below to log a message of an arbitrary (perhaps
+# dynamic) level.
+#
+# 1. Message in block.
+#
+#      logger.fatal { "Argument 'foo' not given." }
+#
+# 2. Message as a string.
+#
+#      logger.error "Argument #{ @foo } mismatch."
+#
+# 3. With progname.
+#
+#      logger.info('initialize') { "Initializing..." }
+#
+# 4. With severity.
+#
+#      logger.add(Logger::FATAL) { 'Fatal error!' }
+#
+# The block form allows you to create potentially complex log messages,
+# but to delay their evaluation until and unless the message is
+# logged.  For example, if we have the following:
+#
+#     logger.debug { "This is a " + potentially + " expensive operation" }
+#
+# If the logger's level is +INFO+ or higher, no debug messages will be logged,
+# and the entire block will not even be evaluated.  Compare to this:
+#
+#     logger.debug("This is a " + potentially + " expensive operation")
+#
+# Here, the string concatenation is done every time, even if the log
+# level is not set to show the debug message.
+#
+# === How to close a logger
+#
+#      logger.close
+#
+# === Setting severity threshold
+#
+# 1. Original interface.
+#
+#      logger.sev_threshold = Logger::WARN
+#
+# 2. Log4r (somewhat) compatible interface.
+#
+#      logger.level = Logger::INFO
+#
+#      DEBUG < INFO < WARN < ERROR < FATAL < UNKNOWN
+#
+#
+# == Format
+#
+# Log messages are rendered in the output stream in a certain format by
+# default.  The default format and a sample are shown below:
+#
+# Log format:
+#   SeverityID, [Date Time mSec #pid] SeverityLabel -- ProgName: message
+#
+# Log sample:
+#   I, [Wed Mar 03 02:34:24 JST 1999 895701 #19074]  INFO -- Main: info.
+#
+# You may change the date and time format via #datetime_format=
+#
+#   logger.datetime_format = "%Y-%m-%d %H:%M:%S"
+#         # e.g. "2004-01-03 00:54:26"
+#
+# Or, you may change the overall format with #formatter= method.
+#
+#   logger.formatter = proc do |severity, datetime, progname, msg|
+#     "#{datetime}: #{msg}\n"
+#   end
+#   # e.g. "Thu Sep 22 08:51:08 GMT+9:00 2005: hello world"
+#
+class Logger
+  VERSION = "1.2.8"
+  ProgName = "#{File.basename(__FILE__)}/#{VERSION}"
+
+  class Error < RuntimeError # :nodoc:
+  end
+  # not used after 1.2.7. just for compat.
+  class ShiftingError < Error # :nodoc:
+  end
+
+  # Logging severity.
+  module Severity
+    # Low-level information, mostly for developers
+    DEBUG = 0
+    # generic, useful information about system operation
+    INFO = 1
+    # a warning
+    WARN = 2
+    # a handleable error condition
+    ERROR = 3
+    # an unhandleable error that results in a program crash
+    FATAL = 4
+    # an unknown message that should always be logged
+    UNKNOWN = 5
+  end
+  include Severity
+
+  # Logging severity threshold (e.g. <tt>Logger::INFO</tt>).
+  attr_accessor :level
+
+  # program name to include in log messages.
+  attr_accessor :progname
+
+  # Set date-time format.
+  #
+  # +datetime_format+:: A string suitable for passing to +strftime+.
+  def datetime_format=(datetime_format)
+    @default_formatter.datetime_format = datetime_format
+  end
+
+  # Returns the date format being used.  See #datetime_format=
+  def datetime_format
+    @default_formatter.datetime_format
+  end
+
+  # Logging formatter, as a +Proc+ that will take four arguments and
+  # return the formatted message. The arguments are:
+  #
+  # +severity+:: The Severity of the log message
+  # +time+:: A Time instance representing when the message was logged
+  # +progname+:: The #progname configured, or passed to the logger method
+  # +msg+:: The _Object_ the user passed to the log message; not necessarily a String.
+  #
+  # The block should return an Object that can be written to the logging device via +write+.  The
+  # default formatter is used when no formatter is set.
+  attr_accessor :formatter
+
+  alias sev_threshold level
+  alias sev_threshold= level=
+
+  # Returns +true+ iff the current severity level allows for the printing of
+  # +DEBUG+ messages.
+  def debug?; @level <= DEBUG; end
+
+  # Returns +true+ iff the current severity level allows for the printing of
+  # +INFO+ messages.
+  def info?; @level <= INFO; end
+
+  # Returns +true+ iff the current severity level allows for the printing of
+  # +WARN+ messages.
+  def warn?; @level <= WARN; end
+
+  # Returns +true+ iff the current severity level allows for the printing of
+  # +ERROR+ messages.
+  def error?; @level <= ERROR; end
+
+  # Returns +true+ iff the current severity level allows for the printing of
+  # +FATAL+ messages.
+  def fatal?; @level <= FATAL; end
+
+  #
+  # === Synopsis
+  #
+  #   Logger.new(name, shift_age = 7, shift_size = 1048576)
+  #   Logger.new(name, shift_age = 'weekly')
+  #
+  # === Args
+  #
+  # +logdev+::
+  #   The log device.  This is a filename (String) or IO object (typically
+  #   +STDOUT+, +STDERR+, or an open file).
+  # +shift_age+::
+  #   Number of old log files to keep, *or* frequency of rotation (+daily+,
+  #   +weekly+ or +monthly+).
+  # +shift_size+::
+  #   Maximum logfile size (only applies when +shift_age+ is a number).
+  #
+  # === Description
+  #
+  # Create an instance.
+  #
+  def initialize(logdev, shift_age = 0, shift_size = 1048576)
+    @progname = nil
+    @level = DEBUG
+    @default_formatter = Formatter.new
+    @formatter = nil
+    @logdev = nil
+    if logdev
+      @logdev = LogDevice.new(logdev, :shift_age => shift_age,
+        :shift_size => shift_size)
+    end
+  end
+
+  #
+  # === Synopsis
+  #
+  #   Logger#add(severity, message = nil, progname = nil) { ... }
+  #
+  # === Args
+  #
+  # +severity+::
+  #   Severity.  Constants are defined in Logger namespace: +DEBUG+, +INFO+,
+  #   +WARN+, +ERROR+, +FATAL+, or +UNKNOWN+.
+  # +message+::
+  #   The log message.  A String or Exception.
+  # +progname+::
+  #   Program name string.  Can be omitted.  Treated as a message if no
+  #   +message+ and +block+ are given.
+  # +block+::
+  #   Can be omitted.  Called to get a message string if +message+ is nil.
+  #
+  # === Return
+  #
+  # +true+ if successful, +false+ otherwise.
+  #
+  # When the given severity is not high enough (for this particular logger), log
+  # no message, and return +true+.
+  #
+  # === Description
+  #
+  # Log a message if the given severity is high enough.  This is the generic
+  # logging method.  Users will be more inclined to use #debug, #info, #warn,
+  # #error, and #fatal.
+  #
+  # <b>Message format</b>: +message+ can be any object, but it has to be
+  # converted to a String in order to log it.  Generally, +inspect+ is used
+  # if the given object is not a String.
+  # A special case is an +Exception+ object, which will be printed in detail,
+  # including message, class, and backtrace.  See #msg2str for the
+  # implementation if required.
+  #
+  # === Bugs
+  #
+  # * Logfile is not locked.
+  # * Append open does not need to lock file.
+  # * If the OS which supports multi I/O, records possibly be mixed.
+  #
+  def add(severity, message = nil, progname = nil, &block)
+    severity ||= UNKNOWN
+    if @logdev.nil? or severity < @level
+      return true
+    end
+    progname ||= @progname
+    if message.nil?
+      if block_given?
+	message = yield
+      else
+	message = progname
+	progname = @progname
+      end
+    end
+    @logdev.write(
+      format_message(format_severity(severity), Time.now, progname, message))
+    true
+  end
+  alias log add
+
+  #
+  # Dump given message to the log device without any formatting.  If no log
+  # device exists, return +nil+.
+  #
+  def <<(msg)
+    unless @logdev.nil?
+      @logdev.write(msg)
+    end
+  end
+
+  #
+  # Log a +DEBUG+ message.
+  #
+  # See #info for more information.
+  #
+  def debug(progname = nil, &block)
+    add(DEBUG, nil, progname, &block)
+  end
+
+  #
+  # :call-seq:
+  #   info(message)
+  #   info(progname,&block)
+  #
+  # Log an +INFO+ message.
+  #
+  # +message+:: the message to log; does not need to be a String
+  # +progname+:: in the block form, this is the #progname to use in the
+  #              the log message.  The default can be set with #progname=
+  # <tt>&block</tt>:: evaluates to the message to log.  This is not evaluated
+  #                   unless the logger's level is sufficient
+  #                   to log the message.  This allows you to create
+  #                   potentially expensive logging messages that are
+  #                   only called when the logger is configured to show them.
+  #
+  # === Examples
+  #
+  #   logger.info("MainApp") { "Received connection from #{ip}" }
+  #   # ...
+  #   logger.info "Waiting for input from user"
+  #   # ...
+  #   logger.info { "User typed #{input}" }
+  #
+  # You'll probably stick to the second form above, unless you want to provide a
+  # program name (which you can do with #progname= as well).
+  #
+  # === Return
+  #
+  # See #add.
+  #
+  def info(progname = nil, &block)
+    add(INFO, nil, progname, &block)
+  end
+
+  #
+  # Log a +WARN+ message.
+  #
+  # See #info for more information.
+  #
+  def warn(progname = nil, &block)
+    add(WARN, nil, progname, &block)
+  end
+
+  #
+  # Log an +ERROR+ message.
+  #
+  # See #info for more information.
+  #
+  def error(progname = nil, &block)
+    add(ERROR, nil, progname, &block)
+  end
+
+  #
+  # Log a +FATAL+ message.
+  #
+  # See #info for more information.
+  #
+  def fatal(progname = nil, &block)
+    add(FATAL, nil, progname, &block)
+  end
+
+  #
+  # Log an +UNKNOWN+ message.  This will be printed no matter what the logger's
+  # level.
+  #
+  # See #info for more information.
+  #
+  def unknown(progname = nil, &block)
+    add(UNKNOWN, nil, progname, &block)
+  end
+
+  #
+  # Close the logging device.
+  #
+  def close
+    @logdev.close if @logdev
+  end
+
+private
+
+  # Severity label for logging. (max 5 char)
+  SEV_LABEL = %w(DEBUG INFO WARN ERROR FATAL ANY)
+
+  def format_severity(severity)
+    SEV_LABEL[severity] || 'ANY'
+  end
+
+  def format_message(severity, datetime, progname, msg)
+    (@formatter || @default_formatter).call(severity, datetime, progname, msg)
+  end
+
+
+  # Default formatter for log messages
+  class Formatter
+    Format = "%s, [%s#%d] %5s -- %s: %s\n"
+
+    attr_accessor :datetime_format
+
+    def initialize
+      @datetime_format = nil
+    end
+
+    def call(severity, time, progname, msg)
+      Format % [severity[0..0], format_datetime(time), $$, severity, progname,
+        msg2str(msg)]
+    end
+
+  private
+
+    def format_datetime(time)
+      if @datetime_format.nil?
+        time.strftime("%Y-%m-%dT%H:%M:%S.") << "%06d " % time.usec
+      else
+        time.strftime(@datetime_format)
+      end
+    end
+
+    def msg2str(msg)
+      case msg
+      when ::String
+        msg
+      when ::Exception
+        "#{ msg.message } (#{ msg.class })\n" <<
+          (msg.backtrace || []).join("\n")
+      else
+        msg.inspect
+      end
+    end
+  end
+
+
+  # Device used for logging messages.
+  class LogDevice
+    attr_reader :dev
+    attr_reader :filename
+
+    def initialize(log = nil, opt = {})
+      @dev = @filename = @shift_age = @shift_size = nil
+      if log.respond_to?(:write) and log.respond_to?(:close)
+	      @dev = log
+      else
+      	@dev = open_logfile(log)
+      	@dev.sync = true
+      	@filename = log
+      	@shift_age = opt[:shift_age] || 7
+      	@shift_size = opt[:shift_size] || 1048576
+      end
+    end
+
+    def write(message)
+      begin
+        if @shift_age and @dev.respond_to?(:stat)
+          begin
+            check_shift_log
+          rescue
+            warn("log shifting failed. #{$!}")
+          end
+        end
+        begin
+          @dev.write(message)
+        rescue
+          warn("log writing failed. #{$!}")
+        end
+      rescue Exception => ignored
+        warn("log writing failed. #{ignored}")
+      end
+    end
+
+    def close
+      begin
+        @dev.close rescue nil
+      rescue Exception
+        @dev.close rescue nil
+      end
+    end
+
+  private
+
+    def open_logfile(filename)
+      if (FileTest.exist?(filename))
+     	open(filename, (File::WRONLY | File::APPEND))
+      else
+       	create_logfile(filename)
+      end
+    end
+
+    def create_logfile(filename)
+      logdev = open(filename, (File::WRONLY | File::APPEND | File::CREAT))
+      logdev.sync = true
+      add_log_header(logdev)
+      logdev
+    end
+
+    def add_log_header(file)
+      file.write(
+        "# Logfile created on %s by %s\n" % [Time.now.to_s, Logger::ProgName]
+      )
+    end
+
+    SiD = 24 * 60 * 60
+
+    def check_shift_log
+      if @shift_age.is_a?(Integer)
+        # Note: always returns false if '0'.
+        if @filename && (@shift_age > 0) && (@dev.stat.size > @shift_size)
+          shift_log_age
+        end
+      else
+        now = Time.now
+        period_end = previous_period_end(now)
+        if @dev.stat.mtime <= period_end
+          shift_log_period(period_end)
+        end
+      end
+    end
+
+    def shift_log_age
+      (@shift_age-3).downto(0) do |i|
+        if FileTest.exist?("#{@filename}.#{i}")
+          File.rename("#{@filename}.#{i}", "#{@filename}.#{i+1}")
+        end
+      end
+      @dev.close rescue nil
+      File.rename("#{@filename}", "#{@filename}.0")
+      @dev = create_logfile(@filename)
+      return true
+    end
+
+    def shift_log_period(period_end)
+      postfix = period_end.strftime("%Y%m%d")	# YYYYMMDD
+      age_file = "#{@filename}.#{postfix}"
+      if FileTest.exist?(age_file)
+        # try to avoid filename crash caused by Timestamp change.
+        idx = 0
+        # .99 can be overridden; avoid too much file search with 'loop do'
+        while idx < 100
+          idx += 1
+          age_file = "#{@filename}.#{postfix}.#{idx}"
+          break unless FileTest.exist?(age_file)
+        end
+      end
+      @dev.close rescue nil
+      File.rename("#{@filename}", age_file)
+      @dev = create_logfile(@filename)
+      return true
+    end
+
+    def previous_period_end(now)
+      case @shift_age
+      when /^daily$/
+        eod(now - 1 * SiD)
+      when /^weekly$/
+        eod(now - ((now.wday + 1) * SiD))
+      when /^monthly$/
+        eod(now - now.mday * SiD)
+      else
+        now
+      end
+    end
+
+    def eod(t)
+      Time.mktime(t.year, t.month, t.mday, 23, 59, 59)
+    end
+  end
+
+
+  #
+  # == Description
+  #
+  # Application -- Add logging support to your application.
+  #
+  # == Usage
+  #
+  # 1. Define your application class as a sub-class of this class.
+  # 2. Override 'run' method in your class to do many things.
+  # 3. Instantiate it and invoke 'start'.
+  #
+  # == Example
+  #
+  #   class FooApp < Application
+  #     def initialize(foo_app, application_specific, arguments)
+  #       super('FooApp') # Name of the application.
+  #     end
+  #
+  #     def run
+  #       ...
+  #       log(WARN, 'warning', 'my_method1')
+  #       ...
+  #       @log.error('my_method2') { 'Error!' }
+  #       ...
+  #     end
+  #   end
+  #
+  #   status = FooApp.new(....).start
+  #
+  class Application
+    include Logger::Severity
+
+    # Name of the application given at initialize.
+    attr_reader :appname
+
+    #
+    # == Synopsis
+    #
+    #   Application.new(appname = '')
+    #
+    # == Args
+    #
+    # +appname+:: Name of the application.
+    #
+    # == Description
+    #
+    # Create an instance.  Log device is +STDERR+ by default.  This can be
+    # changed with #set_log.
+    #
+    def initialize(appname = nil)
+      @appname = appname
+      @log = Logger.new(STDERR)
+      @log.progname = @appname
+      @level = @log.level
+    end
+
+    #
+    # Start the application.  Return the status code.
+    #
+    def start
+      status = -1
+      begin
+	log(INFO, "Start of #{ @appname }.")
+	status = run
+      rescue
+	log(FATAL, "Detected an exception. Stopping ... #{$!} (#{$!.class})\n" << $@.join("\n"))
+      ensure
+	log(INFO, "End of #{ @appname }. (status: #{ status.to_s })")
+      end
+      status
+    end
+
+    # Logger for this application.  See the class Logger for an explanation.
+    def logger
+      @log
+    end
+
+    #
+    # Sets the logger for this application.  See the class Logger for an explanation.
+    #
+    def logger=(logger)
+      @log = logger
+      @log.progname = @appname
+      @log.level = @level
+    end
+
+    #
+    # Sets the log device for this application.  See <tt>Logger.new</tt> for an explanation
+    # of the arguments.
+    #
+    def set_log(logdev, shift_age = 0, shift_size = 1024000)
+      @log = Logger.new(logdev, shift_age, shift_size)
+      @log.progname = @appname
+      @log.level = @level
+    end
+
+    def log=(logdev)
+      set_log(logdev)
+    end
+
+    #
+    # Set the logging threshold, just like <tt>Logger#level=</tt>.
+    #
+    def level=(level)
+      @level = level
+      @log.level = @level
+    end
+
+    #
+    # See Logger#add.  This application's +appname+ is used.
+    #
+    def log(severity, message = nil, &block)
+      @log.add(severity, message, @appname, &block) if @log
+    end
+
+  private
+
+    def run
+      # TODO: should be an NotImplementedError
+      raise RuntimeError.new('Method run must be defined in the derived class.')
+    end
+  end
+end