Probability <- function(N, f, m, b, x, t) {
        #N is the number of lymph nodes
        #f is the fraction of Dendritic cells (in the correct node) that have 
the
antigen
        #m is the number of time steps
        #b is the starting position (somewhere in the node or somewhere in the 
gap
between nodes. It is a number between 1 and (x+t))
        #x is the number of time steps it takes to traverse the gap
        #t is the number of time steps it takes to traverse a node. 
                A <- 1/N
                B <- 1-A
                C <- 1-f
                D <- (((m+b-1)%%(x+t))+1) 

                if (b<=t) {########starts inside node
                        if (m<=(t-b)){return(B + A*(C^m))} # start & end in 
first node          
                        if (D<=t) { # we finish in a node
                                a <- (B + A*(C^(t-b))) #first node
                                b <- ((B + A*(C^t))^(floor((m+b)/(x+t))-1))  # 
intermediate nodes (if
any)
                                c <- (B + A*(C^D))  # last node   
                                d <- (a*b*c)    
                                return(d)
                                } else {Probability(N, f, (m-1), b, x, t)} ## 
finish in a gap   
                } else {###### starts outside node
                        if (m<=(x+t-b)) {return(1)} #also end in the gap
                        if (D<=t) { #end in a node
                                b <- ((B + A*(C^t))^(floor((m/(x+t)))))
                                c <- (B + (A*(C^D)))
                                d <- (b*c)
                                return(d)
                                } else {Probability(N, f, (m-1), b, x, t)} 
#outside node
                }
        }       

I have the following code and I know it works, but I need to explain what is
going on, particularly with the recursion.
Is it true that "when each call finishes - it will pass a quantity back to
the next generation above until you return to the start of the chain, then
outputs the final result."? If so, could someone explain it in a bit more
clearly? if not, how does the recursion work - how does it finally output a
value?

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