Support for Character Entities in RELAX NG

Hello,

The RELAX NG Tutorial, Appendix A [1] mentions that RELAX NG 
"does allow entities to be specified". 

In a XML Schema schema, for example, one could introduce 
character entities via an internal DTD subset or using xsd:notation. 
Could somebody please suggest (a similar or otherwise) an optimal 
way of introducing character entities in a RELAX NG schema?

Thanks.
Pankaj Kamthan

[1] http://www.oasis-open.org/committees/relax-
ng/tutorial.html#c41b1c17 .