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[Yuliang Wang <address@hidden>]

In math mode if one inputs `ab` with no space between the two letters , then 
we get the upright letters `ab`. What is the nature of the string `ab` in 
this case? It's not converted to normal text, as I can tell from the status 
bar. They are still separate math symbols, but typed in upright case (but 
with no `math-up` tag). What's the intention of this face-changing behavior? 
Of course, we can insert a * to get what we want and everything is 
semantically correct. 

I don't know about other subjects, but in mathematics one almost never 
represent a variable by more than one letter. So `ab` means nothing but 
`a*b`. Then why not insert a multiplication symbol automatically so that the 
* will not become the most used key when typing math documents. If I want an 
operator named `ab`, then using the "insert operator" is a better way?

A somewhat related question is the following. There are 3 methods to type 
$\sin x$ in the math mode.

1. Type `sin x`
2. Type `\sin x`
3. Use toolbar icon `op` $\rightarrow$ `Normal`, type `sin`, exit the tag, 
then type space and x. Of course, here we have more options for the typeface.

The appearance of the three are the same and they are all semantically 
correct. The 3rd approach has an additional tag 'math-up' around each letter 
in `sin`. Which one is the correct approach?