When a statement is compiled, {HEAD ~ the compiler} generates a list of dependencies, {TAIL ~ which list} is compared with the identification list 
When a statement is compiled , { HEAD ~ the compiler } generates a list of dependencies , { TAIL ~ which list } is compared with the identification list
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH