Match each form of intervention to what defines it.

• Sole author → Carries out the act personally and alone
• Co-author → Carries out the act jointly with others, sharing control of the event
• Indirect author → Carries out the act through another person used as an instrument
• Inducer → Directly moves another to commit the offence, and is considered an author
• Necessary cooperator → Contributes an act without which the offence would not have been done
• Accomplice → Cooperates by prior or simultaneous acts falling short of the above

The last pair is the one that decides real cases. An accomplice and a necessary cooperator both contribute without controlling the event, and the difference, whether the contribution was indispensable, carries a full degree of penalty. Courts approach it by asking whether the contribution was a scarce good the others could not readily have obtained elsewhere.