rewrite buggy Expression.coefficients() without Maxima

[rwst]

At the moment the ex1.coefficients(ex2) method uses pexpect-Maxima to get a coefficient list of a symbolic expression ex1 when seen as a polynomial in ex2 but the list is wrong whenever the exponent of ex2 is nonnumeric:

sage: var('a, x')
(a, x)
sage: f = a*x - x^x
sage: f.coefficient(x^x)
-1
sage: f.coefficients(x)
[[-x^x, 0], [a, 1]]

It should not be difficult to rewrite coefficients using wildcard power search and the coefficient method (which doesn't use Maxima).

CC: @paulmasson

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/20455

[rwst]

comment:1

For implementation reasons the rewrite happened in #21034 but the behaviour was not changed so this ticket still has to be addressed.

[rwst]

comment:3

This is fixed in current Pynac master.

[rwst]

comment:4

Since #21369 contains doctests for this, it can be closed.