虽然 SVG 上的大部分交互是通过鼠标进行的，但是 SVG 也支持键盘输入。
While most interaction with SVG is through the mouse, SVG also supports keyboard input.
在某些情况下，你需要为习惯于命令行界面的用户设计，他们强烈希望支持键盘输入。
In some cases, you will be designing for users who are accustomed to working on command-line operating systems and the applications made for them. They will be keen to have keyboard support.
您可以使用键盘输入这些表达式，然后观察expr的工作，现在您已经有了足够的知识来修改该计算器代码，以添加支持这些函数的按钮。
You can use the keyboard to type these in and see expr work; and you should have enough knowledge now to modify the calculator code to add buttons that support these functions.
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