I was confused by the statement of the Claim 3.1.

I think I figure it out:

s is (f,t) sorted by t means that:

- the concatenation of f and t, namely, (f,t) is such that s \subset (f,t) and (f,t) \subset s
- s is sorted by t

I was confused by the statement of the Claim 3.1.

I think I figure it out:

s is (f,t) sorted by t means that:

- the concatenation of f and t, namely, (f,t) is such that s \subset (f,t) and (f,t) \subset s
- s is sorted by t

Marking this as solved.

An example is as follows.

Let f be A, E

and let t be A, B, C, D, E

Then s is as follows:

A, A, B, C, D, E, E