In the book "presentation of groups by johnson" in page $36$ they are trying to obtain a group that is locally free but not free (Example $2$).They have proved that the group $U=\bigcup_{n \in \mathbb{N}} F_n$ is hopfian. Upto that I have understood. But I could not understand why the group $U$ is not free. It is described in line 17,18 and 19 on page $36$. Can anyone please explain it? and please refer the book for reference.
Thanks!