Attached is the section on Combinatorial Proofs from my text. I am a little confused. I understand the proof of the special case, then it goes on to say "It is no harder to prove in general."

I would prove this identity in the following manner but I'm not sure if the following is a combinatorial proof or not. Please let me know if it is, or if it is not a combinatorial proof, please point me to a resource so I will understand it better.

Prove the identity:

LHS

RHS

Since the LHS equals the RHS this completes the proof.

Please let me know if the above is a combinatorial proof or not.