[igraph] edge indexing

[Elena Tea Russo]

Good morning/afternoon/evening everybody;

I have a question about the edges' id. 

In particular, I can't understand what happens to edges' id when I add/remove 
egdes and when I transform an undirected graph to a directed (MUTUAL) one.

In the first case, I suppose that, if you have a graph with N edges indexed 
from 0 to N-1 and you add the (N+1)th edge, it will be indexed as N. Am I right 
or am I not?
Then, what happens when I remove an edge? there will be a gap or a re-indexing?

And, in particular: let us suppose i have an undirected graph with N edges, 
indexed from 0 to N-1. I want to transform the graph into a directed one with 
mutual edges, i.e. the number of edges will be doubled. What happens to the 
ids? Let us suppose I consider the edge indexed as j; the mutual edge will be 
j+1 (and thus, the old edges will be re-indexed) or N+j? Or something else?

Thank you for your help!
Best regards

Et Russo