Typesetting annuity and life-insurance symbols in ConTeXt

This is Plain TeX, but I guess it can do for ConTeXt. Experts can improve it.

\def\actuarial#1{%
  \vbox{
    \offinterlineskip
    \tabskip=0pt
    \mathsurround=0pt
    \halign{##&\vrule##\cr
      \noalign{\hrule}%
      &height 1pt\cr
      $\scriptstyle#1$&\cr
    }%
  }%
}

$a_{\actuarial{n}}$

\bye

Based on Barbara Beeton's comment, you just need to pick a font that includes the actuarial bend symbol. For example, using XITS fonts you get:

% Use a math font that has the actuarial bend symbol
\usemodule[simplefonts]
\setmathfont[XITS] 

\Umathchardef\actuarial "0 "0 "20E7

\starttext

$a_{n \actuarial}$

\stoptext

If someone can tell what is the right mathclass and tex name for this glyph, I can send in a request to add this to char-def.lua so that it works out of the box in ConTeXt.