Use Case 5.27 - Integrate Computation Components (MathML)

We have this covered by extension steps.  For example, the following
pipeline computes the kernel for each matrix found in the document by
locating the MathML matrix elements (mtable/matrix) and then running a
custom filter step that computes the kernel and outputs a new mtable/matrix
element.

<?xml version="1.0" encoding="UTF-8"?>
<p:declare-step type="ex:sign" xmlns:p="http://www.w3.org/ns/xproc"
    xmlns:c="http://www.w3.org/ns/xproc-step" version="1.0"
    xmlns:ex="http://www.example.org/math"
    xmlns:m="http://www.w3.org/1998/Math/MathML">
   <p:input port="source"/>
   <p:output port="result"/>
   <p:import href="kernel.xpl"/>
   <p:viewport match="m:math/m:mtable | m:math/matrix">
      <ex:matrix-kernel/>
   </p:viewport>
</p:declare-step>

-- 
--Alex Milowski
"The excellence of grammar as a guide is proportional to the paucity of the
inflexions, i.e. to the degree of analysis effected by the language
considered."

Bertrand Russell in a footnote of Principles of Mathematics