execute (Debugger::ListCommand)

    # File cli/ruby-debug/commands/list.rb, line 16
        def execute
          listsize = Command.settings[:listsize]
          if !@match || !(@match[1] || @match[2])
            b = @state.previous_line ?
            @state.previous_line + listsize : @state.line - (listsize/2)
            e = b + listsize - 1
          elsif @match[1] == '-'
            b = if @state.previous_line
                  if  @state.previous_line > 0
                    @state.previous_line - listsize
                  else
                    @state.previous_line
                  end
                else
                  @state.line - (listsize/2)
                end
            e = b + listsize - 1
          elsif @match[1] == '='
            @state.previous_line = nil
            b = @state.line - (listsize/2)
            e = b + listsize -1
          else
            b, e = @match[2].split(/[-,]/)
            if e
              b = b.to_i
              e = e.to_i
            else
              b = b.to_i - (listsize/2)
              e = b + listsize - 1
            end
          end
          @state.previous_line = display_list(b, e, @state.file, @state.line)
        end