[O] getting calc-units working in table formulas

[Eric Abrahamsen]

I've been playing with calc-units, and it's pretty amazing. See all the
units with `calc-view-units-table'.

Some calc-units stuff works out the box (maybe have to require
calc-units?), I think this should be mentioned in the manual:

| distance | time   | speed       |
|----------+--------+-------------|
| 3 km     | 2.5 hr | 1.2 km / hr |
#+TBLFM: @2$3=$1/$2

Who knew it could do that?! Probably everyone but me... It doesn't need
the constants.el package, and looks nicer in the input, to boot.

calc-units makes a few of its functions available via defmath:

| speed        | simplified speed |
|--------------+------------------|
| 40km / 2.5hr | 16. km / hr      |
|              |                  |
#+TBLFM: @2$2=usimplify($1)

But it's got a lot more tricks. I think unit conversion would be very
handy to have, but there's something I'm not getting about using
defmath. For instance, this works:

| km    |      ft |
|-------+---------|
| 2.5km | 8202.10 |
#+TBLFM: $2='(calc-eval (math-convert-units (calc-eval $1 'raw) (calc-eval "ft" 
'raw))); %.2f

But this doesn't:

#+BEGIN_SRC emacs-lisp
  (defmath uconvert (expr target-units)
    (math-convert-units expr target-units))
#+END_SRC

| km    | ft                  |
|-------+---------------------|
| 2.5km | uconvert(2.5 km ft) |
#+TBLFM: $2=uconvert($1 ft)

I can't tell if I've written the calcFunc thing wrong, or if it's
somehow not getting installed correctly. I've tried several variants,
and they mostly all just give me the results above.

Does anyone know what I'm doing wrong?

Also, once that's figured out, wouldn't it be handy if Org came with a
few predefined units-related math functions? It would be a tiny bit of
code, for quite a bit more power.

At the very least, I'd like to provide a patch to the manual to make the
units stuff a little more explicit...

Eric