[]
[1, 2, 3, 4, 5, 6, 7]
{1, 2, 3}
{1, 2, 3}
⟅1 # 4, 2 # 2, 3 # 2⟆
⟅1 # 7, 2 # 4, 3 # 2⟆
⟅1 # 15, 2 # 4, 3 # 6⟆