Combinatorial chem - Stills binary code tag system

mycotheology

In my med chem book:
http://books.google.ie/books?id=Qe9l...system&f=false
they talk about a method of tagging in which you attach aryl halides to the linker then use gas chromatography to get a "bar code" which tells you what building blocks were used to synthesize each compound. I don't get it at all. Are these aryl halides attached to the link in a sequence or is it just a single aryl halide attached to a linker at a time? How can these aryl halides tell you what building block was used at what stage? For example, in table 5.3 they show you an example of an encoding system and they say T1 for glycine, T2 for alanine and T1 + T2 for serine. How would they attach T1 and T2 to the same resin bead? Does this method only work for tiny libraries? For example how would you apply this method to library of tripeptides made up of 20 different amino acids? You would need loads of different aryl halides.