Intern:Fused ring systems in PANGAEA

This is a work in progress!

=Definition=

=Official nomenclature= The official nomenclature for fused ring systems is prescribed by the International Union of Pure and Applied Chemistry (IUPAC).

Several revisions have been undertaken over the decades. These are the chronologically ordered nomenclature rules on fused ring systems:
 * Fused polycyclic hydrocarbons (1979)
 * Fusion nomenclature (1993)
 * Nomenclature of fused and bridged fused ring systems (1998) [paper format]
 * Fused and bridged fused systems (chapter P-25 in Nomenclature of Organic Chemistry 2013)

=Most important definitions and rules at a glance=
 * Fused ring systems have the maximum number of non-cumulated double-bonds (as many double bonds as possible in an aromatic system)
 * Ortho-fused ring systems: Rings are connected via one shared bond/edge and two atoms. If one ring is connected to two or more other rings, these rings are not located at adjacent bonds!
 * Ortho- and peri-fused ring systems: At least one ring of the system is connected with two or more fused rings at adjacent bonds.
 * Lower case letters (letter locants) in square brackets denote at which bonds of the main ring systems (suffix) another ring system (prefixes) is fused.
 * If technically possible, use square brackets instead of round brackets.
 * Commas between letters locants: Several substituent-rings (prefix) are fused to several non-adjacent bonds of the main ring system (suffix). This is an ortho-fused compound.
 * No commas between letters locants: A substituent-ring (prefix) is fused to several adjacent bonds of the main ring system (suffix). This is an ortho- and peri-fused compound.
 * Periphal numbering: Normal number locants
 * Interior numbering: Number locants + lower case letter (e.g. 2a)
 * If several fusion names are possible depending on chosen system-components, the preferred name should consider the seniority order of parent compounds.

=Abbreviations= No official and consistently used system for abbreviating fused ring systems exist. Many scientists use some sort of acronyms or syllabic abbreviations, others use final clipping to abbreviate fused compounds. However, the abbreviations vary largely. A list of abbreviations used, can be found here.

=Ortho-fused ring systems= Two rings of the system share two common ring atoms (2n) and the bond/edge in between (n)

=Ortho- and peri-fused ring systems= At least one ring of the system is fused with two or more adjacent rings, so that (n) bonds/edges and <(2n) atoms are shared

=Old nomenclatures and deprecated terms=