Consider add-on-the-fly mode for monoidal operators?

Blockly's mutator blocks are actually not all that intuitive for adding many arguments to a block like `&`. We should eventually maybe consider more options for adding new connectors automatically as needed: when all existing slots are full, just add a new one, defaulting to the identity value. Then there will be questions about how to delete slots, insert them in the middle, etc... but maybe they can be solved in a way that's more direct than the mutator dialog.

首先检查 Blockly 的 mutator blocks，以及当前对 `&` 等幺半群运算符的处理方式。确定自动创建 connector、单位元默认值、删除和中间插入应如何结合；由于 issue 没有指定文件或测试，完成标准应是达成一致的设计和实现计划。

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