Add Expression.coefficient(...expand=True) keyword

[rwst]

There is no reason why there should be different results:

sage: (x*(x*y+y^2)).coefficient(x)
y
sage: (x*(x*y+y^2)).coefficient(x,2)
0
sage: (x*(x*y+y^2)).expand().coefficient(x)
y^2
sage: (x*(x*y+y^2)).expand().coefficient(x,2)
y

This would also be unsurprising for Mma users, see #9505 comment:6

Maybe add keyword expand=True.

Component: symbolics

Author: Ralf Stephan

Branch/Commit: u/rws/20824 @ 7ab049f

Issue created by migration from https://trac.sagemath.org/ticket/20824

[rwst]

Description changed:

--- 
+++ 
@@ -11,3 +11,11 @@
 y
 ```
 This would also be unsurprising for Mma users, see [#9505 comment:6](https://github.com/sagemath/sage/issues/9505#comment:6)
+
+There is also
+
+```
+sage: (x^2*y^2).coefficient(x)
+0
+```
+

[rwst]

Description changed:

--- 
+++ 
@@ -12,10 +12,3 @@
 ```
 This would also be unsurprising for Mma users, see [#9505 comment:6](https://github.com/sagemath/sage/issues/9505#comment:6)
 
-There is also
-
-```
-sage: (x^2*y^2).coefficient(x)
-0
-```
-

[rwst]

Description changed:

--- 
+++ 
@@ -12,3 +12,4 @@
 ```
 This would also be unsurprising for Mma users, see [#9505 comment:6](https://github.com/sagemath/sage/issues/9505#comment:6)
 
+Maybe add keyword `expand=True`.

[rwst]

Branch: u/rws/expression_coefficient___problems

[rwst]

Author: Ralf Stephan

[rwst]

Commit: a45909c

[rwst]

New commits:

a45909c

20824: add Expression.coefficient(...expand=True) keyword