Choices

• choice requires exactly one of a group of specified items to appear

• Can be enclosed in optional, oneOrMore, or zeroOrMore

A song must have at least one of ARTIST, COMPOSER, or PRODUCER:

start =
  element SONG {
    element TITLE       { text },
    (element COMPOSER   { personContent }
     | element PRODUCER { personContent }
     | element ARTIST   { text })+,
    element PUBLISHER   { text }?,
    element LENGTH      { text }?,
    element YEAR        { text }?
  }
personContent =
  element NAME {
    element GIVEN    { text }
    & element FAMILY { text }
  }