\underparen - Used to put a (stretchy) under-parenthesis (under-arc, smile) under the argument.
{ \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}}