func insert_after(a,b) {
    b{:next} = a{:next};
    a{:next} = b;
}

var B = Hash.new(
    data => 3,
    next => nil,    # not a circular list
);
var A = Hash.new(
    data => 1,
    next => B,
);
var C = Hash.new(
    data => 2,
);

insert_after(A, C);