I just read the definition of a divisor of an elliptic curve  and I'm not able to understand the syntax. So my questions are targeting the following steps:
1.) What are formal symbols, specially in that case?
2.) How are those coeefficient determined? Is there any "ahh, I see" example?
3.) About what can I think, if I'm confronted with divisors? Are those a kind of an ideal? I know, that they perform an additive group, but I'm not sure, >>how<< to think.
 http://people.cs.nctu.edu.tw/~rjchen/ECC2009/22_Divisor.pdf p. 3, def. 1.1