Hello.
I want to write a function for reducing an
expression for an element x \in sl2 into a sum in the
Poincare Birkhoff Witt basis , that is,

e,f,h=symbols('efh',commutative=False)

e*f |---> f*e+h

I tried to use subs naively and it wouldn't work:

>>>e,f,h=symbols('efh',commutative=False)

>>> (e*f).subs(e*f,f*e+h)
h + f*e
>>> (e*f*e*f).subs(e*f,f*e+h)
e*f*e*f

I tried to check the help(Symbol) and I couldn't find anything useful.
I would like to know if there is a good way to do it.

Thank you very much.

K.

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