Element Declarations
A model group is used to define an element that has mixed content or element content.
A model group is bounded by brackets, and contains at least one token.
When a model group contains more than one content token, the child elements are controlled using two logical connector operators; sequence connector ‘,’, and choice connector ‘|’
<!ELEMENT element1 (a, b, c)> indicates a is followed by element b, which in turn is followed by c.
<!ELEMENT element2 (a | b | c)> indicates either one can be selected.
Combinations are possible: (a,b,(c|d)), or ((a,b,c) | d)