A substring is a contiguous block of symbols within a string. Equivalently, a string 
is a substring of 
iff 
for some strings 
and 
,
either of which may be empty (CRC Press LLC 2004).
For example, bc is a substring of abcd, whereas ac is a subsequence of its symbols but is not a substring. The term subword permits nonconsecutive symbols in the convention used for the Bruhat order. A substring of a word is also called a factor.
