Its sole purpose is to present some concepts in {HEAD ~ a simplified form} as {TAIL ~ a prelude} to the more detailed description that is discussed later 
as
Its sole purpose is to present some concepts in { HEAD ~ a simplified form } as { TAIL ~ a prelude } to the more detailed description that is discussed later
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH