We say that stringst and u can be interwoven into a larger string s if there
is a substring s′ of s whose symbols are formed of t and u as disjoint subsequences;
that is, every symbol of s′ must appear in t or u, and the same symbol of s′ cannot
derive from both t and u.
For example, the strings "ACAG" and "CCG" can be
interwoven into "GACCACGGTT".
However, they cannot be interwoven into "GACCACAAAAGGTT"
because of the appearance of the four 'A's in the middle of the subsequences.