Lua filter to insert non-breable spaces

["krulis....-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org" <krulis.tomas.tk-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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I have the same situation as person asking here on discuss: 
<https://groups.google.com/g/pandoc-discuss/c/tZ2f51iLEIk/m/CSF_AWUXBQAJ>
I am trying to insert non-breakable spaces instead of specific Space 
elements in text. I tryed to work it out from example "Removing spaces 
before citations;" but I am probably using Inlines wrong.

I am asking this, because it seems that in the original question has no 
final answer/solution.

The filter I am using is as this:

```nonbreakableSpace.lua
function myCheckString(myString)
  if myString == 'a' or
  myString == 'i' then
    return true
  else
    return false
  end
end

function Inlines (inlines)
  for i = 1, #inlines do
    if inlines[i].t == 'Space' then
      if inlines[i - 1].t == 'Str' then
        local oneLetterPrefix = myCheckString(inlines[i - 1].c)
        if oneLetterPrefix == true then
          inlines[i - 1] = inlines[i - 1].c .. "\160" .. inlines[i + 1].c
          inlines:remove(i)
          inlines:remove(i + 1)
        end
      end
    end
  end
  return inlines
end
```

and very short test file:

```test.txt
a\ i test

a i test
```

Most probably it is evident that I am lua newbie. But the idea is to take 
list of Inlines, check every element if it is type Space, then check if 
previous element is Str, and then file check this strings contents, if it 
matches `myString` (names are horrible, I know). 

So the expected native pandoc out of the test is:

[Para [Str "a\160i\160test"]
Para [Str "a\160i\160test"]]

However, currently I am getting error:

nonbreakableSpace .lua:12: attempt to index a nil value (field '?')
stack traceback:
        pandocVlna.lua:12: in function 'Inlines'

How could I fix that?

Regards, Tomas

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