Mention .coerce() method in coercion tour

src/doc/en/tutorial/tour_coercion.rst

@@ -280,6 +280,8 @@ we have:
     x
     sage: R2(y)
     y
+    sage: R2.coerce(y)
+    y
 
 If there is no name preserving ring homomorphism, coercion is not
 defined. However, conversion may still be possible, namely by mapping
@@ -296,6 +298,12 @@ ring generators according to their position in the list of generators:
     z
     sage: R3(y)
     x
+    sage: R3.coerce(y)
+    Traceback (most recent call last):
+    ...
+    TypeError: no canonical coercion
+    from Multivariate Polynomial Ring in x, y over Integer Ring
+    to Multivariate Polynomial Ring in z, x over Integer Ring
 
 But such position preserving conversions do not qualify as coercion:
 By composing a name preserving map from ``ZZ['x','y']`` to ``ZZ['y','x']``

src/doc/fr/tutorial/tour_coercion.rst

@@ -279,6 +279,8 @@ des variables. Nous avons donc :
     x
     sage: R2(y)
     y
+    sage: R2.coerce(y)
+    y
 
 En l'absence d'un morphisme d'anneau qui préserve les noms de variable, la
 coercition entre anneaux de polynômes multivariés n'est pas définie. Il peut
@@ -296,6 +298,12 @@ celle de l'autre en fonction de leur position dans la liste des générateurs :
     z
     sage: R3(y)
     x
+    sage: R3.coerce(y)
+    Traceback (most recent call last):
+    ...
+    TypeError: no canonical coercion
+    from Multivariate Polynomial Ring in x, y over Integer Ring
+    to Multivariate Polynomial Ring in z, x over Integer Ring
 
 Mais une telle conversion ne répond pas aux critères pour être une coercition :
 en effet, en composant l'application de ``ZZ['x','y']`` dans ``ZZ['y','x']``

src/doc/ja/tutorial/tour_coercion.rst

@@ -252,6 +252,8 @@ Sageが宗とするのは歩み寄りだ．
     x
     sage: R2(y)
     y
+    sage: R2.coerce(y)
+    y
 
 
 変数名を維持する環準同形写像が定義できなければ，型強制も成立しない．
@@ -268,6 +270,12 @@ Sageが宗とするのは歩み寄りだ．
     z
     sage: R3(y)
     x
+    sage: R3.coerce(y)
+    Traceback (most recent call last):
+    ...
+    TypeError: no canonical coercion
+    from Multivariate Polynomial Ring in x, y over Integer Ring
+    to Multivariate Polynomial Ring in z, x over Integer Ring
 
 ところが，そうした順序依存の変換は型強制としては満足すべきものにならない．
 ``ZZ['x','y']`` から ``ZZ['y','x']`` への変数名維持写像と ``ZZ['y','x']`` から ``ZZ['a','b']`` への順序依存写像を合成すると，結果は変数名も順序も保存しない写像となって無矛盾性が破れてしまうからである．

src/doc/pt/tutorial/tour_coercion.rst

@@ -285,6 +285,8 @@ preservam nomes. Então temos:
     x
     sage: R2(y)
     y
+    sage: R2.coerce(y)
+    y
 
 Se não existir homomorfismo de anel que preserve nomes, coação não é
 definida. Todavia, conversão pode ainda ser possível, a saber,
@@ -302,6 +304,12 @@ geradores:
     z
     sage: R3(y)
     x
+    sage: R3.coerce(y)
+    Traceback (most recent call last):
+    ...
+    TypeError: no canonical coercion
+    from Multivariate Polynomial Ring in x, y over Integer Ring
+    to Multivariate Polynomial Ring in z, x over Integer Ring
 
 Mas essas conversões que preservam a posição não se qualificam como
 coação: Compondo um mapa que preserva nomes de ``ZZ['x','y']`` para