$$\underset{\genfrac{}{}{0ex}{}{\mathrm{first\; under\; script}}{\mathrm{second\; one}}}{\mathrm{BASE}}=\underset{\phantom{\mathrm{fail}}}{\underset{\genfrac{}{}{0ex}{}{\mathrm{first\; under\; script}}{\mathrm{second\; one}}}{\mathrm{BASE}}}$$