Slightly adapted from a suggestion by Scott Pakin, here is LaTeX code to generate the "suppose" symbol used by some mathematicians in writing proofs:
\newlength{\Swidth}`
\newcommand{\suppose}{%
\settowidth{\Swidth}{S}
\makebox[\Swidth]{\(\_{\rule{.15ex}{.8ex}}\)}\llap{S}%
}
``Suppose there exists some \(x\) in set \(U\ldots\)'' becomes
\(\suppose\ \exists\ x\in U\ldots\)
The symbol is the letter S with a vertical bar through the bottom curve:
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