code: @Override
      public ValNums apply(Env env, Env.StackHelp stk, AstRoot asts[]) {
          Frame fr = stk.track(asts[1].exec(env)).getFrame();
          boolean narm = asts[2].exec(env).getNum() == 1;
          double[] ds = new double[fr.numCols()];
          Vec[] vecs = fr.vecs();
          for (int i = 0; i < fr.numCols(); i++)
              ds[i] = kurtosis(vecs[i], narm);
          return new ValNums(ds);
      }
docs: (kurtosis ary na.rm)