Membership of a DS is determined as follows. An atom is added to the DS by setting its selected attribute to true through a process called selection. As noted previously (see: Atoms and Bonds), only some atomic states are compatible with selection. A bond is added to the DS if both of the atoms it spans are selected and the bond itself is elided. This allows arbitrary bonds between selected atoms to be excluded from the DS by ensuring they are not elided. A bond to a bridge node will be added to the DS only if both the parent and the referenced atom are selected.