Page #4, Example 10-1:
[Example 10-1]
1 cmd> 1 _history: {}
<enter> cmd> _history: { “1” } // add to history
2 cmd> 2 _history: { “1” }
<enter> cmd> _history: { “1”, “2” } // add to history
<enter> cmd> _history: { “1”, “2” } // no change, just a new line
<up arrow> cmd> 2 _history: { “1”, “2”, “” } // retrieving history
0 cmd> 20 _history: { “1”, “2”, “” }
<enter> cmd> _history: { “1”, “2”, “20” } // replace tmp
原先第七行 _history 漏了 ""，请自行更正补上。