public static @Nullable PsiElement getCellStart(@NotNull PsiElement element) {
    PsiElement el = element;
    while (el != null && !isBlockCell(el)) {
      el = PsiTreeUtil.prevLeaf(el);
    }
    if (el == null) {
      return element.getContainingFile().getFirstChild();
    }
    return PsiTreeUtil.nextLeaf(el);
  }
public static @Nullable PsiElement getCellStart(@NotNull PsiElement element)