In the following sentence, can we leave out articles? If we can, what's the grammar rule?
John Lennon, (an) English singer, (a) musician, was born in Liverpool.
I read a sentence in Advanced Grammar in Use by Martin Hewings:
Mr. Bob Timms, leader of the Democratic Party, MP for Threeoaks, has announced his resignation.
There are no articles before the words, 'leader' and 'MP', so I wonder what the difference is.