texmath-0.6.7: tests/writers/munder-10.tex
\underset{\mathrm{long\ under\ script}}{\mathrm{B}}=\underset{\mathrm{long\ under\ script}}{\phantom{\mathrm{x}}\mathrm{B}\phantom{\mathrm{x}}}\underset{\mathrm{long\ under\ script}}{\mathrm{B}}=\underset{\mathrm{long\ under\ script}}{\phantom{\mathrm{x}}\mathrm{B}\phantom{\mathrm{x}}}