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ConTeXt has a nice structure called mathstackers in order to define new math commands in which some characters are stacked over another one. For more information refer to math-stc.mkiv.

The two command \mathover[symbol]{}{} and \mathunder[symbol]{}{} are built and can be used directly. However one may define one's own commands.

A typical example is when you want to put a small circle on top of a letter (as one would when defining the topological interior of a set…). To do so we define first a new stacker named here MySymbol, and then invoking that with \definemathover we define two new commands \interior and \mystarred which can be used in math mode.

	topoffset=.4\mathemwidth, % poor man's italic correction
\definemathover[MySymbol][interior]["2218] %this a small circle
\definemathover[MySymbol][mystarred]["2717] % this is an asterisk

Here is the new command \type{\interior{K}}: $\interior{K}$. Compare this to \type{\mathring{K}}: $\mathring{K}$.

Here is the new command \type{\mystarred{K}}: $\mystarred{K}$.

Here are the built-in commands \type{\mathover} and \type{\mathunder}: 

$\mathover[symbol]{"2217}{A}$, % 2217 is an asterisk
$\mathover[symbol]{"2218}{A}$, % 2218 is a circle
$\mathover[symbol]{"2219}{A}$, % 2219 is a bullet


When typeset in mkiv one can see that the command \interior{K} shows the letter K with a small circle above it.