Assgning a value to one field, how can I make the other fields changing.

Consider the following R5 class

C<-setRefClass("C", 
      fields=list(a="numeric",b="numeric")
      , methods=list(
      seta = function(x){
      a<<-x
      b<<-x+10
      cat("The change took place!")
      }
      ) # end of the methods list
      ) # end of the class

Now create the instance of the class

c<-C$new() 

This command

c$seta(10)

will result in that c$a is 10 and c$b is 20.

So it actually works, however, I want to achieve this result by the command

c$a<-10

(i.e. after that I want c$b to be equal to 20 as defined in the class in the logic of seta() function)
How can I do it?