json

<!DOCTYPE BankData [
  <!ELEMENT BankData (Customer•, Account•)>
  <!ELEMENT Customer (Name, SSN, Phone+)>
    <!ATTLIST Customer
      customerID ID #REQUIRED
      owns IDREFS #IMPLIED
    >
  <!ELEMENT Name (#PCDATA)>
  <!ELEMENT SSN (#PCDATA)>
  <!ELEMENT Phone (#PCDATA)>
  <!ELEMENT Account (Type, Balance)>
  <!ATTLIST Account
    accountID ID #REQUIRED
    ownedBy IDREFS #IMPLIED
  >
  <!ELEMENT Type (#PCDATA)>
  <!ELEMENT Balance (#PCDATA)>
]>

alternatively we can model Type as an attribute of Account,
then we can make it an enumerated type.

<!ATTLIST Account type (checking|saving) #REQUIRED>