Returns a vector of valid edge ids (corresponding to non-NULL edges) for a network that may have some deleted edges.
a network object, possibly with some deleted edges.
The edge ids used in the network package are positional indices on the internal "mel" list. When edges are removed using
NULL elements are left on the list. The function
valid.eids returns the ids of all the valid (non-null) edge ids for its
a vector of integer ids corresponding to the non-null edges in x
If it is known that x has no deleted edges,
seq_along(x$mel) is a faster way to generate the sequence of possible edge ids.
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