This class provides a way to store the Contracts taken by the
agent, and means to know if a contract is already taken on
specified resources and who is the initiator of these contracts.
this method is called by the strategy to send a retractation
for contracts that are less important to their initiator if the
current contract has been taken.
this method is called by the strategy to send a retractation
for contracts that are less important to their initiator if the
current contract has been taken.