number and type of locks for an alter sequence

Hello ,

In PG10, when I am altering a sequence, i see 2 locks in pg_locks matching
my sequence:
a RowExclusiveLock
and a ShareRowExclusiveLock

My question is Why 2 locks?
Someone on irc said it was for catalog but i am no sure because
pg_locks.relationid is the same for the 2 locks.

At the same time, If I am trying to call nextval with an othersession, the
session wants to get a RowExclusiveLock on the sequence.

So, to me, the lock ShareRowExclusiveLock is when the sequence is beeing
altered.
And the RowExclusiveLock is for concurent access between alter/drop
sequence and nextval/setval/curval/lastval.

Documentation seems not talking about this... (
https://www.postgresql.org/docs/10/explicit-locking.html) Maybe should I
open a ticket?

Thanks