10.83.3 Can you answer these questions?

10.83.3.1 Is there a reliable detection routine for finding out if a given single token can be used as a^\237..control sequencea^\237(c) of a a^\237..definitiona^\237(c)?
10.83.3.2 Unwanted indent with bidi and tcolorbox packages
10.83.3.3 Checking if a TeX definition already exists
10.83.3.1 Is there a reliable detection routine for finding out if a given single token can be used as a^\237..control sequencea^\237(c) of a a^\237..definitiona^\237(c)?

https://tex.stackexchange.com/landing/r/digest?cta=question&id=733403

The following questions are "academical" ones. The questioner does not have a "real life task" in mind which cannot be accomplished without what is asked for. 
TeXbook, Chapter 24: ... 
[macros] [tex-core] [definition]
 
asked by Ulrich Diez https://tex.stackexchange.com/landing/r/digest?cta=user&id=118714 Score of 3
10.83.3.2 Unwanted indent with bidi and tcolorbox packages

https://tex.stackexchange.com/landing/r/digest?cta=question&id=733247

Note: Everything below is compiled with XeLaTeX. 
I'm trying to develop pullout boxes for a textbook using the tcolorbox package. Sometimes these boxes will contain multiple paragraphs, for which I ... 
[xetex] [tcolorbox] [overleaf] [bidi]
                                                                                  

                                                                                  
 
asked by neruneru https://tex.stackexchange.com/landing/r/digest?cta=user&id=365654 Score of 1
10.83.3.3 Checking if a TeX definition already exists

https://tex.stackexchange.com/landing/r/digest?cta=question&id=733311

I know, that there are other solutions. 
Question is: How may I get the #1 into the place of the dots in this example? 
\TeX 
\def\check#1{\ifx#1\undefined ... is undefined\else now ... is ... 
[plain-tex] [ifundefined]
 
asked by MBE https://tex.stackexchange.com/landing/r/digest?cta=user&id=284656 Score of 1