 Example:
   curl -u sk_abcdefQWERTY090900000000: -H "Content-Type: application/json" \
     -d '{"account_ids":["cc3cb7f1-14f6-4a18-a803-5034af8c0004","9b896055-920a-413c-9172-dfb4007a8e7f"]}' \
-    https:// cocalc.com/api/v1/get_usernames
+    https://cocalc.com/api/v1/get_usernames
   ==>  {"event":"usernames",
         "id":"32b485a8-f214-4fda-a622-4dbfe0db2b9c",
         "usernames": {

    error         : false

# client --> hub
API message
    event             : 'change_email_address'
    id                : undefined
    account_id        : required
    old_email_address : ""        # ignored -- deprecated
    new_email_address : required
    password          : ""

# hub --> client
message

Option 'agreed_to_terms' must be present and specified as true.
Account creation fails if there is already an account using the
given email address, if 'email_address' is improperly formatted,
and if password is fewer than 6 or more than 64 characters.

Attempting to create the same account a second time results in an error:
  curl -u sk_abcdefQWERTY090900000000: \
