A symmetric relationship is one whose type is symmetric (its own inverse). An example of a symmetric
relationship is synonymy (since, if a is a synonym of b, then be is a synonym of a). Symmetric relationships
differ from asymmetric relationships in that there is no definite divergence point between the ancestry of
the source and target synsets. Another way of saying this is that the target synset will always be in
the source's ancestry, and vice versa. For this reason, symmetric relationships have no concept of a
common parent index.