I want to know about the "Completion amalgams". I know the definition, but I don't know how to find the completion amalgams for example: for an amalgam $\displaystyle (S_n, S_n, S_{n-1})$.

If you know a good book for the amalgams pls let me know. Still I couldn't find a book.

Pls help me to find the answer at least for the example.

Thanx.