How can i call Python/Shell from a macro ?

Hi,

I am able to write simple macros in the preamble.

I wish to define a macro that takes in a TeXmacs text and does some 
computations with
Python (or other, actually my favorite is Ruby) then prints out the 
result of Python execution.

I have tried to copy from what: Insert -> Fold -> Executable -> Python 
does, but unsuccessfully.
<script-input|python|default|123+1>    => nope, does not run as I expect.

My aim is this, i have a lot of very easy calculation steps in an 
engineering document, i wish
to be able to write the expression to compute e.g. $123.4_[m] / 1.2_[m]$ 
then call the macro above it
and have it become $123.4_[m] / 1.2_[m]$ = 
<RESULT_OF_PYTHON_EATING_THE_FORMULA>.

It could be fine for me to call the shell.

At the moment I think every call to the Shell/Python/xxx can be 
independent, that is, i
don't need/want to memorize previous calculations outcome to do the next.


bye
Nicola