letter key的用法和样例：
例句
- After it opens, pressing the appropriate letter key executes the function.
助记符模式打开以后，按下适当的字母键就可以执行相应的功能。 - To compact data by combining parts of the data, for example, to transform a two-word alphabetic key into a one-word numeric key by adding the numeric equivalents of the letters.
通过将数据的某些部分进行组合来压缩数据，例如，通过将一些字母的数字等值相加，可把两个字长的字母键词变换为一个字长的数字键词。
