Compute counting variable in dataframe

I have an R data frame:

a <- 1:12  
list <- c(rep("x",3),rep("y",4),rep("z",3),rep("x",2))  
data <- data.frame(a,list)

data  
 a list  
 1    x  
 2    x  
 3    x  
 4    y  
 5    y  
 6    y  
 7    y  
 8    z  
 9    z  
10    z  
11    x  
12    x

I want to create a new column which begins counting at 1 every time the value of "list" changes, i.e. in this example:

b <- c(1:3,1:4,1:3,1:2)    
data <- data.frame(a,list,b)  

I am far from being an expert in R and cannot for the life of me work out an efficient way of doing this. My main problem seems to be that any value of "list" can come back at any time, but there is no rule to the length of the blocks of one value. Does anyone have any ideas? Thanks!