Actually, thinking about this a bit more, maybe it makes sense to adopt your suggested insertion/deletion definitions, and translate simple indels that are represented like GCG -> GCGCG in the VCF to a canonical representation where one side is always a single base (i.e. "G->GCG" for the example, but with position incremented by 2), when we convert from a VCF. Probably better to deal with that in a different pull request though.