Check if a word is inside another word.

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You may use the method is_factor:

sage: w = Word('11')
sage: w.is_factor(Word('011'))
True
sage: w.is_factor(Word('101'))
False

To find the first occurrence, you need to reverse the arguments because it searches the input in self:

sage: Word('011').first_occurrence(w)
1
sage: Word('101').first_occurrence(w) is None
True

Or using find as in Python method find for str which returns -1 if not found:

sage: Word('011').find(w)
1
sage: Word('101').find(w)
-1