Thus quoth  [email protected]  at 01:44 on Sat, Feb 08 2014:
>
> i follow the definition define theta in sympy, however got error at g = 
> CompositeMorphism(f, f) - IdentityMorphism(A)

This is because the subtraction operator is not overloaded for *Morphism
classes.  I am not sure what you are trying to achieve with this code,
by the way: in my version of "Conceptual Mathematics" I can see no
"morphism subtractions" on the page you cite.

-- 
Sergiu

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