The situation is more complex in the case of fused rings. For example, a bicyclic fused ring system consists of three cycles: two smaller ones, and one larger one around the perimeter. This means that, depending on the traversal order of the SMILES string, even if there is a six-membered ring the digit 6 may not be present as a ring closure symbol in DeepSMILES. Given that the traversal order is arbitrary when writing SMILES, it is worth considering whether there is a preferred traversal order that would make it easier to learn the resulting SMILES or DeepSMILES. For example, if one follows shorter branches first, then the number of parentheses will be reduced. We note that this aspect of SMILES may be less of an issue if a random traversal order is used to generate one or more SMILES for each structure ( , , ).