filter_backtrace

# File lib/irb/workspace.rb, line 89
    def filter_backtrace(bt)
      case IRB.conf[:CONTEXT_MODE]
      when 0
        return nil if bt =~ /\(irb_local_binding\)/
      when 1
        if(bt =~ %/tmp/irb-binding! or
            bt =~ %irb/.*\.rb! or
            bt =~ /irb\.rb/)
          return nil
        end
      when 2
        return nil if bt =~ /irb\/.*\.rb/
        return nil if bt =~ /irb\.rb/
      when 3
        return nil if bt =~ /irb\/.*\.rb/
        return nil if bt =~ /irb\.rb/
        bt = bt.sub(/:\s*in `irb_binding'/, '')
      end
      bt
    end