/usr/share/texlive/texmf-dist/tex/generic/genmisc/undertilde.tex is in texlive-generic-recommended 2013.20140215-1.
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1 2 3 | \def\undertilde#1{\mathord{\vtop{\ialign{##\crcr
$\hfil\displaystyle{#1}\hfil$\crcr\noalign{\kern1.5pt\nointerlineskip}
$\hfil\tilde{}\hfil$\crcr\noalign{\kern1.5pt}}}}}
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