The empty word is a factor of a word

[seblabbe]

The following results should be True.

sage: Word().is_factor(Word())
False
sage: Word().is_factor(Word('abad'))
False
sage: Word().is_factor(Word([0,1,2]))
False
sage: Word('').is_factor(Word('abad'))
False
sage: Word([]).is_factor(Word([0,1,2]))
False

CC: @sagetrac-sage-combinat @sagetrac-abmasse

Component: combinatorics

Keywords: empty word

Author: Sébastien Labbé

Reviewer: Alexandre Blondin Massé

Merged: sage-4.3.3.alpha0

Issue created by migration from https://trac.sagemath.org/ticket/8215

[seblabbe]

Attachment: trac_8215_empty_word-sl.patch.gz

[seblabbe]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
-The following three results should be True.
+The following results should be True.
 
 ```
 sage: Word().is_factor(Word())

[seblabbe]

Author: Sebastien Labbe

[sagetrac-abmasse]

Reviewer: Alexandre Blondin Massé

[sagetrac-abmasse]

comment:2

Tested on sage 4.3.1. Doc builds fine, all tests passed and it fixes the bug. Not much more to say... Positive review !

[qed777]

Merged: sage-4.3.3.alpha0