The map tells the computer which atoms are which between the
reactant and product sides of the reaction.
Atom maps are a 'class' property for the atoms in the reaction,
excluding the agent atoms.
Values are numbers, without intrinsic meaning. The system
may renumber and reorder them.
An atom map is optional. Can be omitted, or only partially included.
No requirements for completeness, "chemical sensibility".
Independent of the underlying connectivity.