Reactions are made up of one or more molecules. The molecules are expressed with the SMILES language. Any valid SMILES can be a component molecule in a reaction.
Two new operators are introduced to SMILES. The '>' separates the molecules into groups, based on their role in the reaction (eg. reactant, agent, or product). The '+' operator separates individual molecules within the parts of the reaction.
All parts are optional. ('>>' is a valid reaction)
Agents are defined as molecules which do not contribute or accept atoms in the overall reaction, although for reactions where the atom map is not specified, this is not enforced. (eg. 'C=C>BrBr>BrCCBr' is legal)
Note that "+" and "." are not interchangeable.