该组合键无效。组合键必须由零或多个组合键和一个单键组成。
This key combination is not valid. Key combinations must consist of zero or more modifier keys and a single key.
单元测试：给一个无效的PHP数组键不打破。可能吗?。
Unit Testing: Give an invalid array key that doesn't break PHP. Possible?
无效的关系定义：不同长度的键。
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