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+1 vote

A single question mark indicates a pattern (or wildcard), so that should only be used in e.g. substitute. Under most circumstances you don't need them for normal indices. Two question marks indicate an object wildcard, that is, a wildcard which matches an entire tensor. Have a look at


in the reference guide, and then reply to this answer in case it is still not clear.

answered by (44.9k points)

I was checking the notebook, and it is much understandable than the last time I read it (couple of years ago).

Nonetheless, right at the (current) end of the notebook, there are missing explanations:
- What is the meaning and use of Q::.?
- Why the first terms of the expression following the Q::. assignation are not shown?
- You used a \Q and a Q in the mentioned expression. Where are defined and what's their difference?

Thank you!