Hello,
I'm not sure if I should open a ticket on the following issue, or if
this is just a case of "Error Between Keyboard And Chair". Here is a
simplified version of what happened.
I had parents A and B and map phi between them, as follows:
sage: A = CombinatorialFreeModule(ZZ,[1,2,3],prefix='X'); X = A.basis()
sage: B = CombinatorialFreeModule(ZZ,[4,5,6],prefix='Y'); Y = B.basis()
sage: phi = A.module_morphism(lambda i: Y[i+3],codomain=B)
sage: phi(X[1] + X[2]) # example
Y[1] + Y[2]
I wonder if there is already a coercion from A and B:
sage: B.has_coerce_map_from(A)
False
There isn't, so I decide to add one.
sage: phi.register_as_coercion()
AssertionError: coercion from Free module generated by {1, 2, 3} over
Integer Ring to Free module generated by {4, 5, 6} over Integer Ring
already registered or discovered
But that seems not to be true:
sage: B.has_coerce_map_from(A)
False
***
It seems that the first call to 'has_coerce_map_from' touches the
coercion cache, and as a result it is no longer possible to register a
coercion between the associated objects. But it took me some time to
work out what was actually going on; I did not expect asking the
question to forbid me from providing an answer! So my first thought was
that this is a bug. On the other hand, I don't know much about the
coercion system or why this behavior exists, so if it really is intended
to work this way, I won't open a ticket. (Or maybe just a documentation
ticket).
Thanks for any explanation,
Jason
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