将表应用到向量列表并将其聚合

I have this:

我有这个:

require(data.table)
items = list(c(1,1,3), c(2,2,4), c(3,4,5,6))
multiplier = c(10,20,30)
dt = data.table(items, multiplier)
#     items multiplier
#1:   1,1,3         10
#2:   2,2,4         20
#3: 3,4,5,6         30
require(data.table)
item