Slices by length

I quite often find myself writing expressions of the form
     someString[x : x+n]
where n is often an int and x may be an int, a variable, or a (possibly 
complicated) expression.
It would be more natural to be able to specify the slice not by its 
startpoint and ENDPOINT,
but by its startpoint and LENGTH, particularly when x is a long 
expression and has to be repeated or pre-stored in a variable.
I have worked with languages that had this facility, and I miss it.
The possible syntaxes are restricted by the need to avoid ambiguity with 
existing valid code.
It could be for example
     someString[x ! n]
(of course, x and n could be arbitrary expressions).
One point to be decided would be whether negative n would produce
     (a) an empty string, so that s[x ! n] was always the same as s[x : 
x+n]
     (b) a slice ending at x, e.g. "01234567"[4!-3] would equal "123" 
(or less plausibly "321").
I don't have a strong opinion on this; there may be good reasons for 
preferring one to another.
Does anybody think this is a good idea?
Best wishes
Rob Cliffe
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