allow post-creation (pre-use) declaration of coercions

[robertwb]

One can now do

a = A()
mor = [morphism from X to A]
A.register_coercion(mor)

This works as long as coercion has not yet been used (otherwise information cached (both here and elsewhere) becomes invalid).

CC: @nthiery

Component: coercion

Author: Robert Bradshaw

Reviewer: Nick Alexander

Merged: sage-4.2.alpha1

Issue created by migration from https://trac.sagemath.org/ticket/5598

[robertwb]

comment:1

Attachment: 5598-coerce-declare.patch.gz

Depends on #5597.

[robertwb]

comment:2

One can now do

sage: A = SomeParent()
sage: mor = [morphism from X to A]
sage: A.register_coercion(mor)

This works as long as coercion has not yet been used (otherwise information cached (both here and elsewhere) becomes invalid).

[robertwb]

Description changed:

--- 
+++ 
@@ -1,7 +1,9 @@
 One can now do 
 
+```
 a = A()
 mor = [morphism from X to A]
-a.register_coercion(mor)
+A.register_coercion(mor)
+```
 
 This works as long as coercion has not yet been used (otherwise information cached (both here and elsewhere) becomes invalid). 

[robertwb]

comment:5

Actually, though I wrote it on top of #5597, there's probably not a dependance. Note that I don't have any doctests yet (as it's completely unused functionality at this point).

[williamstein]

comment:6

Note that I don't have any doctests yet (as it's completely unused
functionality at this point).

Hence I'm physically incapable of giving this a positive review. Sorry.