Children are added to an atom through the sequence non-terminal ("sequence"). A sequence starts with a required atom non-terminal. If an allowed non-terminal does not follow, the corresponding atom will have no children. Allowed non-terminals will be one of: union; branch; or gap. The union non-terminal consists of an optional bond non-terminal followed by a mandatory non-terminal selected from bridge or sequence. If either of these latter non-terminals are detected but bond is not, the resulting bond is elided.