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NODE SYMBOLS USED IN STN CHEMICAL STRUCTURE BUILDING

        
                  Symbol              Substitution Allowed at the Site
        
                  element symbol      only that element
                  X                   any halogen (F, Cl, Br, I, At)
                  M                   any metal (1)
                  -                   excludes the element or group which 
                                      follows the minus sign
                  Q                   any element except C or H
                  A                   any element except H
                  Gk (k=1-20)         user-defined variable groups (2)
                  Ak                  any carbon chain          
                  Cy                  any cyclic group
                  Cb                  any carbocyclic group
                  Hy                  any heterocyclic group
                  shortcut symbols
(1) CAS defines a metal as any element except Ar As At B Br C Cl F H He I Kr N Ne O P Rn S Se Si Te Xe.
(2) A special command, either VAR (variable) or REP (repeating) is used to define Gk groups.