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I mean , that 's the storybook , man . For instance , whenever a prime p occurs in the sequence , it is preceded by 2p and followed by 3p . Although much about headaches still is n't understood , some researchers think migraines may be caused by functional changes in the trigeminal nerve system , a major pain pathway in your nervous system , and by imbalances in brain chemicals , including serotonin , which regulates pain messages going through this pathway . He expects the decline to continue until the government reaches a surplus in 2012 , three years after he has left office . When Achilles had covered that tenth , the tortoise would have covered an additional one-hundredth , and so on . Incirlik was the northern base for warplanes enforcing no-fly rules against Saddam Hussein 's Iraq after the 1991 Persian Gulf War .