Fixed point of word morphism is broken on some tuple input #8595
Description
From sage-combinat-devel group :
2010/3/23 Vincent Delecroix
> Hi,
>
> I tried the following and get an unexpected error
>
> {{{
> sage: s = WordMorphism({'a1': ['a1','a2'], 'a2':['a1']})
> sage: s.fixed_point('a1')
> Traceback
> ...
> KeyError: 'a'
> }}}
>
> and it does the same for tuples
>
> {{{
> sage: s = WordMorphism({('a', 1) : [('a', 1), ('a', 2)], ('a', 2) : [('a', 1)]})
> sage: s.fixed_point(('a', 1))
> Traceback
> ...
> KeyError: 'a'
> }}}
>
> Is it a bug or not the right way to do ?
>
> (On this example it looks strange but I really need product alphabet)
>
> Cheers,
> Vincent
CC: @videlec
Component: combinatorics
Author: Sébastien Labbé
Reviewer: Vincent Delecroix
Merged: sage-4.4.alpha0
Issue created by migration from https://trac.sagemath.org/ticket/8595