bug#66714: [FIXED] Expr substr on plus symbol

[petabaud51]

NEVERMIND, IT'S ON 'INFO EXPR'.
Could you folks please add it to 'man expr'?

XXX
Greetings,
I'm not sure if this is the intended UNIX/POSIX behaviour, but on:
< expr substr a 1 2
, I get:
> a
, which is right, but on:
< expr substr + 1 2
I get:
> expr: syntax error: missing argument after ‘2’

On expr "$line_of_text" 1 2, this error is thrown if the line is a 
simple '+'.
A real-world scenario is getting the first character of each line in 
bulk, crashing if the line is '+'.

< expr --version
> expr (GNU coreutils) 8.32

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