execute (Debugger::WhereCommand)

    # File cli/ruby-debug/commands/frame.rb, line 160
        def execute
          (0...@state.context.stack_size).each do |idx|
            if idx == @state.frame_pos
              print "--> "
            else
              print "    "
            end
            print_frame(idx)

          end
          if truncated_callstack?(@state.context, Debugger.start_sentinal)
            print "Warning: saved frames may be incomplete; compare with caller(0).\n"
          end
        end