element [1:1]
{
    name = people
    type = people
}

element [1:1]
{
    name = person
    type = person
}

people = element [0:-1]
{
    name = person
    type = person
}

person = sequence [1:1]
{
    element [1:1]
    {
        name = name
    }
    element [0:1]
    {
        name = birthdate
    }
    element [0:1]
    {
        name = gender
    }
    element [0:1]
    {
        name = socialsecuritynumber
    }
}