def filter_backtrace(bt)
      case IRB.conf[:CONTEXT_MODE]
      when 0
	return nil if bt =~ /\(irb_local_binding\)/
      when 1
	if(bt =~ %r!/tmp/irb-binding! or
	   bt =~ %r!irb/.*\.rb! or
	   bt =~ /irb\.rb/)
	  return nil
	end
      when 2
	return nil if bt =~ /irb\/.*\.rb/
      when 3
	return nil if bt =~ /irb\/.*\.rb/
	bt.sub!(/:\s*in `irb_binding'/){""} 
      end
      bt
    end
def filter_backtrace(bt)
      case IRB.conf[:CONTEXT_MODE]
      when 0
	return nil if bt =~ /\(irb_local_binding\)/
      when 1
	if(bt =~ %r!/tmp/irb-binding! or
	   bt =~ %r!irb/.*\.rb! or
	   bt =~ /irb\.rb/)
	  return nil
	end
      when 2
	return nil if bt =~ /irb\/.*\.rb/
      when 3
	return nil if bt =~ /irb\/.*\.rb/
	bt.sub!(/:\s*in `irb_binding'/){""} 
      end
      bt
    end
error message manipulator