How to share custom entity definitions across documents

[Taro Ikai <tikai@ABINITIO.COM>]

I want to share entity definitions across multiple Docbook documents.

Is there a way to do this? For example, I want to have:

  <!ENTITY mystring "My String">

in one place, and be able to reference it by &mystring; from many 
documents.

How can I do this? Do I have to modify the DTD? 

Taro