How to handle MathCAD/Mathematica-like text?

Many engineers and academics need to be able to
write active text documents, where there's text
interspersed with mathematical equations, charts,
and so on... and where when they change a number,
the equation results, charts, etc., can change.
MathCAD and Mathematica do this sort of thing.

It appears to me that this format gets rather
close to supporting this.  But it's not clear
that it can _quite_ do it You can use MathML to
represent the equations in a pretty way, and
formulas to express how to compute it.
And the formulas need to be able to connect
to each others... it's not clear how to do that
inside a document.

--- David A. Wheeler