Patterns mention other Patterns and terms before they appear

There are two issues I noticed with the order of patterns in the book:
1. The Module section mentions the Singleton Pattern, which is not covered for two sections:

> "We’ve seen why the Singleton pattern can be valuable"
1. The Observer section mentions the term _Concrete_. This is not explicitly defined until Command Patterns, three sections later.

> "Concrete classes are best explained in terms of class-based programming languages"

Changing the location of / reference to these terms will improve the presentation of the ideas.