Actions define what to do if a signature matches. Currently, there is only one
action defined: event string raises a signature_match
event. The event handler has the following type:
event signature_match(state: signature_state, msg: string, data: string)
See \f{fig:signature-state} for a description of signature_state. The given string
is passed as msg, and data is the current part of the payload that
has eventually lead to the signature match (this may be empty for signatures without
content conditions).