A phonetic choice (even though I don't really know the grammatical rule for the use of a, or, an). intergril - a typo, or, have I been misspelling the word all my life? (My English speaking life, that is...)
...we use ...a... if the next word begins with the sound of a consonant... ...we use...an....if it sounds like a vowel... It's all about the sound and the flow.