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



"[[]][]]"
 
%acc 0 !acc
%flag false !flag
 
len [
    get tochar dup
    "[" == [$acc 1 + !acc] if
    "]" == [$acc 1 - !acc] if
    $acc 0 < [true !flag] if
    ] for
 
print
 
$acc 0 # $flag or ( [" is NOT ok"] [" is OK"] ) if
print
 
" " input