This is TeX, Version 3.14159265 (TeX Live 2019) (preloaded format=tex 2019.5.8)  20 JUN 2019 13:21
**\underline{text}

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               {text}$
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*\underline{$text$}
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              {$text$}
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                text$}
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<*> \underline{$text$}
                      
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*\underline$no$
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               no$
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               no$
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               2$
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*\def\ta{\underline}

*$\ta{x_2}$

*$\underline{x_2}$

*$\underline{2}$

*$\underline{text}$

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