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")))