Add together a list of matrices element-by-element [duplicate]

Answer 1

 > Reduce("+", m1.l)
      [,1] [,2] [,3] [,4]
 [1,]    1    0    0    0
 [2,]    1    1    0    0
 [3,]    1    1    1    0
 [4,]    0    1    1    1
 [5,]    0    0    1    1
 [6,]    0    0    0    1
 [7,]    0    0    0    0
 [8,]    0    0    0    0
 [9,]    0    0    0    0

This will add the first two together, then it will take the result and add the 3rd matrix to it. Then it will add that result to the fourth matrix, etc. until it's worked its way through the whole list.