Task:

 Generate a string with   N   opening brackets   [   and with   N   closing brackets   ],   in some arbitrary order. 
 Determine whether the generated string is balanced; that is, whether it consists entirely of pairs of opening/closing brackets (in that order), none of which mis-nest.


Examples

   (empty)      OK
   []           OK   
   [][]         OK   
   [[][]]       OK 
   ][         NOT OK
   ][][       NOT OK
   []][[]     NOT OK



brack(s) := block(
   [n: slength(s), r: 0, c],
   catch(
      for i thru n do (
         if cequal(c: charat(s, i), "]") then (if (r: r - 1) < 0 then throw(false))
         elseif cequal(c, "[") then r: r + 1
      ),
      is(r = 0)
   )
)$
 
brack("");
true
 
brack("[");
false
 
brack("]");
false
 
brack("[]");
true
 
brack("][");
false
 
brack("[[][]]");
true
 
brack("[[[]][]]]");
false