Maybe we should also indicate this in the syntax.

required <….> optional.

Existing Wording:

The shorthand syntax to define an inline extension is:

• {extension element path}

  contains

  {extensionSliceName1}
  {card1}
  {flags1}

  and

  {extensionSliceName2}
  {card2}
  {flags2}

  ...
  In both styles, the cardinality is required, and flags are optional. Adding an extension below the root level is achieved by giving the full path to the extension array to be sliced.