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



balanced(s)={
  my(n=0,v=Vecsmall(s));
  for(i=1,#v,
    if(v[i]==91,
      n++
    ,
      if(v[i]==93 && n, n--, return(0))
    )
  );
  !n
};
rnd(n)=Strchr(vectorsmall(n,i,if(random(2),91,93)))
forstep(n=0,10,2,s=rnd(n);print(s"\t"if(balanced(s),"true","false")))