A recursive definition of arguments
1. If ? is a sentence, then ? is an argument
If A is an argument and ? a sentence, then {A} ? ? is an argument
If {A1} ? ?, {A2} ? ?, …, {An} ? ? are arguments, then {A1, A2, …, An} ? ? is an argument
? Note: In CumulA a similar definition is used, but allowing composite reasons