The map tells the computer which atoms are which between the
reactant and product sides of the reaction.
The map is essentially a 'hint' about the underlying
reaction mechanism. The mechanism may or may not be reflected in
the connectivity changes over 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 (and does) renumber and reorder them.
Very similar to ring-bond digits: they are interpreted on input,
within the toolkit they are 'invisible', the system will generate them
An atom map is optional. Can be omitted, or only partially included.
No requirements for completeness, "chemical sensibility".
Independent of the underlying connectivity.