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