Why does the musig() descriptor accept the pk descriptor as an argument?

Question

In the following test vector described in BIP390, the musig() descriptor takes a pk() descriptor as an argument:

tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*,pk(f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9))

In the specification, the musig() descriptor takes a Key expression as an argument. I think that the pk descriptor itself is not the Key expression itself. Is this a mistake with the Test Vector?

Answer 1

The pk() is part of the tr() as a script expression. The musig() component is completed at the first comma.