From 3a476bd1ea5aef3ecd375e71d50342f1441fd35d Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Tue, 9 Jun 2020 06:25:01 -0400
Subject: [PATCH] eja: re-disable base ring to algebra coercion.

---
 mjo/eja/eja_algebra.py | 43 +++++++++++++++++++++++++++++++++---------
 1 file changed, 34 insertions(+), 9 deletions(-)

diff --git a/mjo/eja/eja_algebra.py b/mjo/eja/eja_algebra.py
index 26b7248..689a3db 100644
--- a/mjo/eja/eja_algebra.py
+++ b/mjo/eja/eja_algebra.py
@@ -26,13 +26,30 @@ lazy_import('mjo.eja.eja_subalgebra',
 from mjo.eja.eja_utils import _mat2vec
 
 class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule):
-    # This is an ugly hack needed to prevent the category framework
-    # from implementing a coercion from our base ring (e.g. the
-    # rationals) into the algebra. First of all -- such a coercion is
-    # nonsense to begin with. But more importantly, it tries to do so
-    # in the category of rings, and since our algebras aren't
-    # associative they generally won't be rings.
-    _no_generic_basering_coercion = True
+
+    def _coerce_map_from_base_ring(self):
+        """
+        Disable the map from the base ring into the algebra.
+
+        Performing a nonsense conversion like this automatically
+        is counterpedagogical. The fallback is to try the usual
+        element constructor, which should also fail.
+
+        SETUP::
+
+            sage: from mjo.eja.eja_algebra import random_eja
+
+        TESTS::
+
+            sage: set_random_seed()
+            sage: J = random_eja()
+            sage: J(1)
+            Traceback (most recent call last):
+            ...
+            ValueError: not a naturally-represented algebra element
+
+        """
+        return None
 
     def __init__(self,
                  field,
@@ -149,15 +166,22 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule):
             True
 
         """
+        msg = "not a naturally-represented algebra element"
         if elt == 0:
             # The superclass implementation of random_element()
             # needs to be able to coerce "0" into the algebra.
             return self.zero()
+        elif elt in self.base_ring():
+            # Ensure that no base ring -> algebra coercion is performed
+            # by this method. There's some stupidity in sage that would
+            # otherwise propagate to this method; for example, sage thinks
+            # that the integer 3 belongs to the space of 2-by-2 matrices.
+            raise ValueError(msg)
 
         natural_basis = self.natural_basis()
         basis_space = natural_basis[0].matrix_space()
         if elt not in basis_space:
-            raise ValueError("not a naturally-represented algebra element")
+            raise ValueError(msg)
 
         # Thanks for nothing! Matrix spaces aren't vector spaces in
         # Sage, so we have to figure out its natural-basis coordinates
@@ -1886,7 +1910,8 @@ class BilinearFormEJA(FiniteDimensionalEuclideanJordanAlgebra, KnownRankEJA):
 
     SETUP::
 
-        sage: from mjo.eja.eja_algebra import BilinearFormEJA
+        sage: from mjo.eja.eja_algebra import (BilinearFormEJA,
+        ....:                                  JordanSpinEJA)
 
     EXAMPLES:
 
-- 
2.43.2