Re: Prononciation of a/an.
PS Also an unstressed written 'e' can often be realized as /ɪ/ - as in /prɪ'zju:m/. In fact that /ɪ/ is quite common - even giving rise to misspellings (with 'i'). Only yesterday, for example (/ɪg'za:mpǝl/ !), in this forum, I saw 'aggravate' misspelt with an 'i'. (And in that case, as it happens, the misspelling allows for a change in the meaning - for me, 'aggravation' will always involve the idea of making something more grave; but not many people insist on this more restricted meaning; and I have to accept that 'aggravation' can often be used to refer to behaviour that is just disruptive.)
b