Looking at LINQ

I've been digging through the discussion of LINQ in the Albahari brothers' C# 4.0 in a Nutshell: The Definitive Reference, and it has cleared up a number of points for me:

1. At a compiler level, LINQ is a series of transformations done to expressions that begin with FROM and end with SELECT or GROUP BY.  These transformations turn free text queries into extension methods.  For example, var query = from A in aList select A.ToUpper() is transformed by the compiler into var query = aList.Select(a => a.ToUpper()). But, and this is a key point, the implementation of the methods is not determined.  You determine the implementation by the namespaces you include; you could write your own implementation of Select, Where, etc.