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{ \underparen #1}
\underparen command puts a (stretchy) under-parenthesis (under-arc, smile) under the argument.
\underparen a \quad
\underparen ab \quad 
\underparen{ab} \quad
\underparen{abc} \quad
\underparen{abcdef} \quad
\underparen{\overparen{abcd}}