expression parse tree API?

Hello,

could smb point me how to access the formula parse tree.

I  asked in SAGE forum, but seems they don't deal with expression  parsing
http://groups.google.com/group/sage-support/browse_thread/thread/96686f9ff4ba3711
SAGE folks referenced Maxima..

if it is possible, I'd like to reach it from python, but if not, I could 
learn some LISP

---
what I really want, is to make derivative exercise tool.
which would do the differentiation of (complex) formulas step by step.

for example  Maple has it
http://www.adeptscience.co.uk/products/mathsim/maple/whatsnew/maple_presentation.html#MapleAutoBookmark9 


ctrl+F: ShowSteps
or http://www.google.lt/search?q=Student[Calculus1]+ShowSteps

I'd like to have parse-tree structure, based on differentiation by
variable (x) ,
each next depth level would mean a step according to
http://en.wikipedia.org/wiki/Chain_rule

so terminal/final nodes should have 'x' or plain numeric expression,
and pre-terminal nodes should have simple differentiatable functions
a subset of http://en.wikipedia.org/wiki/Table_of_derivatives,
which contain x and numeric values/variables

I found sth similar what I want in java..
http://www.singularsys.com/jep/doc/html/advanced.html

Thanks in advance
-- 
Jurgis Pralgauskis
Don't worry, be happy :) and make things better ;)