R help - Jul 2014 - List of Lists by for Loop

Dear Experts,
I have a one more doubt about making list of lists.
Here is the simple code i have made.
I am doing the following for only one digit=20
a=vector(20,mode="list")
b=vector(20,mode="list")
for (i in 1:20){
  #Do Calculation
  a[[i]]=data.frame(####)
  b[[i]]=data.frame(####)
}

Now i have to repeat the whole process for Digits=c(30,40,50,60)
In short I want the Master list containing following
List 1(#20)
     List a
     List b

List 2(#30)
     List a
     List b

........so on
Can you please guide me in this ?
Thank you very much.
Sincerely

-- 
Munjal Patel

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