More strange [[ behaviour

Hi,

setClass("MyList", "list")

[1] "MyList"

a <- new("MyList")
a

An object of class "MyList"
list()

setMethod("[[", "MyList", function(x, i, j, ...) cat("Just testing\n"))
a[[]]

Just testing

a[[1]]

Just testing

a[[a=1]]

Just testing

a[[b=1]]

Just testing
...

a[[v=1]]

Just testing

a[[w=1]]

Just testing

a[[x=1]]

Error in a[[x = 1]] : subscript out of bounds

a[[y=1]]

Just testing

a[[z=1]]

Just testing

Can someone explain me why the "[[x=1]]" case is treated differently?

Cheers,
H.