tweak_backtrace (Spec::Runner::BacktraceTweaker)

    # File lib/spec/runner/backtrace_tweaker.rb, line 20
          def tweak_backtrace(error)
            return if error.backtrace.nil?
            tweaked = error.backtrace.collect do |message|
              clean_up_double_slashes(message)
              kept_lines = message.split("\n").select do |line|
                ignored_patterns.each do |ignore|
                  break if line =~ ignore
                end
              end
              kept_lines.empty?? nil : kept_lines.join("\n")
            end
            error.set_backtrace(tweaked.select {|line| line})
          end