OpenMath XML (source)
<OMOBJ xmlns="http://www.openmath.org/OpenMath" version="2.0">
<OMA><OMS cd="logic1" name="implies"/>
<OMA><OMS cd="logic1" name="and"/>
<OMA><OMS cd="relation1" name="eq"/>
<OMA><OMS cd="linalg4" name="rowcount"/>
<OMV name="A"/>
</OMA>
<OMV name="m"/>
</OMA>
<OMA><OMS cd="relation1" name="eq"/>
<OMA><OMS cd="linalg4" name="columncount"/>
<OMV name="A"/>
</OMA>
<OMV name="n"/>
</OMA>
</OMA>
<OMA><OMS cd="relation1" name="eq"/>
<OMA><OMS cd="linalg1" name="matrix_selector"/>
<OMA><OMS cd="linalg6" name="matrix_tensor"/>
<OMV name="A"/> <OMV name="B"/>
</OMA>
<OMA><OMS cd="arith1" name="plus"/>
<OMV name="i"/>
<OMA><OMS cd="arith1" name="times"/>
<OMA><OMS cd="arith1" name="minus"/>
<OMV name="k"/><OMI>1</OMI>
</OMA>
<OMV name="m"/>
</OMA>
</OMA>
<OMA><OMS cd="arith1" name="plus"/>
<OMV name="j"/>
<OMA><OMS cd="arith1" name="times"/>
<OMA><OMS cd="arith1" name="minus"/>
<OMV name="l"/><OMI>1</OMI>
</OMA>
<OMV name="n"/>
</OMA>
</OMA>
</OMA>
<OMA><OMS cd="arith1" name="times"/>
<OMA><OMS cd="linalg1" name="matrix_selector"/>
<OMV name="A"/>
<OMV name="i"/><OMV name="j"/>
</OMA>
<OMA><OMS cd="linalg1" name="matrix_selector"/>
<OMV name="B"/>
<OMV name="k"/><OMV name="l"/>
</OMA>
</OMA>
</OMA>
</OMA>
</OMOBJ>
Strict Content MathML
<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply><csymbol cd="logic1">implies</csymbol>
<apply><csymbol cd="logic1">and</csymbol>
<apply><csymbol cd="relation1">eq</csymbol>
<apply><csymbol cd="linalg4">rowcount</csymbol><ci>A</ci></apply>
<ci>m</ci>
</apply>
<apply><csymbol cd="relation1">eq</csymbol>
<apply><csymbol cd="linalg4">columncount</csymbol><ci>A</ci></apply>
<ci>n</ci>
</apply>
</apply>
<apply><csymbol cd="relation1">eq</csymbol>
<apply><csymbol cd="linalg1">matrix_selector</csymbol>
<apply><csymbol cd="linalg6">matrix_tensor</csymbol><ci>A</ci><ci>B</ci></apply>
<apply><csymbol cd="arith1">plus</csymbol>
<ci>i</ci>
<apply><csymbol cd="arith1">times</csymbol>
<apply><csymbol cd="arith1">minus</csymbol><ci>k</ci><cn>1</cn></apply>
<ci>m</ci>
</apply>
</apply>
<apply><csymbol cd="arith1">plus</csymbol>
<ci>j</ci>
<apply><csymbol cd="arith1">times</csymbol>
<apply><csymbol cd="arith1">minus</csymbol><ci>l</ci><cn>1</cn></apply>
<ci>n</ci>
</apply>
</apply>
</apply>
<apply><csymbol cd="arith1">times</csymbol>
<apply><csymbol cd="linalg1">matrix_selector</csymbol><ci>A</ci><ci>i</ci><ci>j</ci></apply>
<apply><csymbol cd="linalg1">matrix_selector</csymbol><ci>B</ci><ci>k</ci><ci>l</ci></apply>
</apply>
</apply>
</apply>
</math>
Popcorn
linalg4.rowcount($A) = $m and linalg4.columncount($A) = $n ==> linalg1.matrix_selector(linalg6.matrix_tensor($A, $B), $i + ($k - 1) * $m, $j + ($l - 1) * $n) = linalg1.matrix_selector($A, $i, $j) * linalg1.matrix_selector($B, $k, $l)