Question: why "index/fill" should exist?

These days I'm learning "maple introductory programming guide". About the command 'anames()',there is an example illustrated in the picture. I know anames(alluser) yields all the user-assigned names,including names containing a leading underscore ('_').

But I don't know why it yields the word "index/fill", can you tell me the meaning of "index/fill" and why it should exist in the result? 

Best regards!


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