Semicircle as an accent mark above a character

I need to write a macro (something like \semicircle{{#1}} ) to place an 
"inverted semicircle" symbol
as an accent mark to be placed over one or two characters, perhaps using 
a \stackrel command  such as the following.
\documentclass{article}
\newcommand{\semicircle}[1]{\ensuremath{\stackrel{\frown}{\mbox{#1}}}}
\begin{document} \semicircle{A}, \semicircle{AB}
\end{document}
The "\frown" symbol here is too flat, and if I use the "\cap" symbol, it 
is too tight,
so the choice is an inverted semicircle sitting over the character(s).
The semicircle should spread over all the characters (one or two) that 
are placed for #1 in the macro -
something like \semicircle{X} or \semicircle (XY} etc.
  Is TikZ a good choice? Any help in building such a macro will be 
appreciated.
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