static int cond (LexState *ls) {
  /* cond -> exp */
  expdesc v;
  expr(ls, &v);  /* read condition */
  if (v.k == VNIL) v.k = VFALSE;  /* 'falses' are all equal here */
  luaK_goiftrue(ls->fs, &v);
  return v.f;
}