Math/stackers

\mathover, \mathunder and \definemathstackers

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.

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

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.

\definemathstackers[MySymbol]
	[voffset=-.3\mathexheight,
	hoffset=\zeropoint,
	mathclass=ord,
	topoffset=.4\mathemwidth, % poor man's italic correction
	middlecommand=\mathematics]
  
\definemathover[MySymbol][interior]["2218] %this a small circle
\definemathover[MySymbol][mystarred]["2717] % this is an asterisk
\starttext

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
$\mathunder[symbol]{"2217}{A}$,
$\mathunder[symbol]{"2218}{A}$,
$\mathunder[symbol]{"2219}{A}$.


\stoptext

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

Overbraces and underbraces

In math mode, you can put a brace over or under an expression this way:

\startformula
a + \overbrace{b + c} + \underbrace{d + e} + \doublebrace{f + g} + h
\stopformula

The same goes for brackets (or other stackers):

\startformula
a + \overbracket{b + c} + \underbracket{d + e} + \doublebracket{f + g} + h
\stopformula

Adjusting stackers

You can adjust where the stackers start and end with the lt (left-top), rt (right-top), lb (left-bottom) and rb (right-bottom) parameters. This feature was introduced in beta 2019-10-28.

$a + \overbrace  [lt=1em]         {b+c+d} + e$\blank
$a + \underbrace [lt=1em]         {b+c+d} + e$\blank
$a + \overbrace  [rb=1em]         {b+c+d} + e$\blank
$a + \underbrace [rb=1em]         {b+c+d} + e$\blank
$a + \overbrace  [lt=1em,rb=1em]  {b+c+d} + e$\blank
$a + \underbrace [lt=1em,rb=1em]  {b+c+d} + e$\blank

Interleaved braces

Some terms of an expressions can be shared by both braces, tuning the lt, rt, lb and rb parameters:

$a + \doublebrace[lt=1.4em,rt=-.3em]                   {b+c+d} + e$\blank
$a + \doublebrace[rb=1em]                              {b+c+d} + e$\blank
$a + \doublebrace[lt=1.4em,rt=-.3em,lb=-.3em,rb=1.5em] {b+c+d} + e$\blank
$a + \doublebrace[lt=-1.6em,rb=-1.4em]                 {b+c+d} + e$\blank

See also

• Chapter "1. Math stackers" in the About manual.
• Section "1.8 Accents" in the Still manual.
• Section "10.6 Fancy fences" in the Math-MkIV manual.