两个list；如何把对应的行列相加？多谢！
list a
$`1`
named integer(0)

$`2`
integer(0)

$`3`
[1] 5

$`4`
named integer(0)

$`5`
[1] 5

$`6`
[1]  13  18  94 111



list b
$`1`
[1] 0

$`2`
[1] 7

$`3`
[1] 67

$`4`
[1] 87

$`5`
[1] 95

$`6`
[1] 106