Multiple Merge data.frame and Calculate the columns

a=cbind("A"=sample(1:100,5), "B"=sample(1:50,5), "a"=rnorm(5), "e"=rnorm(5))
b=cbind("A"=sample(1:100,10),"B"=sample(1:50,10),"b"=rnorm(10),"e"=rnorm(10))
c=cbind("A"=sample(1:100,15),"B"=sample(1:50,15),"c"=rnorm(15),"e"=rnorm(15))
d=cbind("A"=sample(1:100,20),"B"=sample(1:50,20),"d"=rnorm(20),"e"=rnorm(20))

L = list(a,b,c,d)
merge(L[[1]],L[[2]],by=c("A"))
merge(L[[1]],L[[2]],by=c("A","B"))
merge(L[[1]],L[[2]],by=c("A","B"),all=T)

MultiMerge = function(L, by){
  M=L[[1]]
  for(i in 2:length(L))
    M = merge(M,L[[i]],by=by,all=T)
  return(M)
}
M=MultiMerge(L,by=c("A","B"))

dataCol = c("e.")
M = data.frame(M[,-grep(dataCol, colnames(M))], "e" = rowSums(M[,grep(dataCol, colnames(M))], na.rm=T))