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