Arithmetic on strings

[Dirk Laurie <dirk.laurie@...>]

It seems to be impossible to override arithmetic operators on
strings.  One can for example set the __add metamethod, but
it only gets called when at least one of the strings cannot be
coerced to a number.  For example:

   getmetatable''.__add = function(x,y) return x..y end
   'a'+'b' --> 'ab', fine
   '1'+'2' --> 3