@Override
ValFrame apply(Env env, Env.StackHelp stk, AST asts[]) {
Frame fr = stk.track(asts[1].exec(env)).getFrame();
double frac = asts[2].exec(env).getNum();
double nrow = fr.numRows() * frac;
Vec vecs[] = fr.vecs();
long[] idxs = new long[fr.numCols()];
int j = 0;
for (int i = 0; i < idxs.length; i++) if (vecs[i].naCnt() < nrow) idxs[j++] = i;
Vec vec = Vec.makeVec(Arrays.copyOf(idxs, j), null, Vec.VectorGroup.VG_LEN1.addVec());
return new ValFrame(new Frame(vec));
}
