For example...

<!ELEMENT a (b+) >
<!ELEMENT b EMPTY>
<!ELEMENT c (#PCDATA)>
<!ELEMENT a (b,c) >
<!ELEMENT a (b|c)* >
<!ELEMENT a (#PCDATA|b|c)* >
<!ELEMENT a (b,  (c|d)*) >
<!ELEMENT a (b?, (c|d)+) >
<!ELEMENT a (b?, (c+|d+)) >