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>It seems like
>an unnecessary extension of Lua's beautiful tiny grammar, an exception
>that brings minor advantage but carries with it hidden disadvantages.
>Among these are a deterrent to other language modifications and the
>less-than-obvious-without-the-BNF 'f "A" .. "B"' is 'f("A") .. "B"'.

Not quite.
   f "A" .. "B"
will raise a syntax error, presumably to reduce confusion.

The shortcut syntax for tables in particular is the basis of a lot of object
orientated schemes - it hides the function call in a nice little table. Eg a
scheme might work like this:

local newclass = class{
  inherits = someotherclass,

  repaint = function()

and allow instancing of a class like this:
  local window1 = Window{ width = 10, height = 20 }

Obviously that's just one way to implement classes, but you'll find
tables-as-sole-argument pop up a lot if you browse different oo schemes for lua.
The parenthesis would make the schemes look far less like a language built-in,
which is the illusion they currently provide.

It logically follows to support strings too, eg
  print "Hello"

To see strings as sole arguments in style, just take a peek at RiciLake's AsciiMenu:

It would be a great shame, and a very current-program-breaking one to disallow
implicit sole arguments..

- Alex