Establishing a list of the factors of a given length

During the pseudo-algorithm (see PlanForAGeneralStrategy) we often need to provide a list of the factors of a given length (usually small). This can be done algorithmically :

• Input : (a non-erasing substitution), (a letter such that begins with and has at least 2 letters), (an integer).
• Output : List of factors of length at most of the fixed point of that begins with the letter .

1. (liste that contains the word '' as unique element, i.e. )
2. (pointer)
3. while is less or equal to the length of do
4.         
5.          Add to the factors of length at most of that are not yet elements of
6. done
7. Return

Of course line 5 is very imprecise and there are many ways to optimize it (see the last section of http://www.lirmm.fr/~monteil/m2/Monteil-FacteursMotsSubstitutifs.pdf).

A naive implementation is available in SAGE: language of a substitution
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