References
1.International Association for the Study of Pain (1994) Classification of 
chronic pain (2nd edn), Part III: Pain terms, A current list with definitions 
and notes on usage.Campbell, JK, Penzine DB, Wall EM (2000) Evidence-based guidelines 
for migraine headache: Behavioral and physical treatments.The US 
Headache Consortium, American Academy of Neurology, USA.Wikipedia (2018) Pain management.2.4.5.6.7.8.9.