I was trying to understand the example 3 of this paper of Baer.
Take a free group $F$ of rank 2. Take the commutator group $F'$ (this is a free group having infinite countable rank). Then say $N$ the commutator $[F, F'']$.
My question is the following (pag. 273 of the paper): how can we say that $F''/N$ is a free abelian group?
Of course it is a quotient of a free abelian group but I do not understand why it is free...
I know that I can work out the example also without know this fact, I just want to know how he says that; any hints?