There are two kinds of tuples: up tuples and down tuples. We write tuples as ordered lists of their components; a tuple is delimited by parentheses if it is an up tuple and by square brackets if it is a down tuple.
— Structure and Interpretation of Classical Mechanics
So is really a dot product:
— Me@2019-12-14 06:11:22 PM
2019.12.14 Saturday (c) All rights reserved by ACHK