I don't get this working with mutable. If I do

    val = mutable(GValue())
    ccall((:gtk_tree_model_get_value,libgtk), Void, (Ptr{GObject},Ptr{GtkTreeIter},Cint,Ptr{GValue}),
           treeModel, &iter, column-1, val)
    val[]

the return value is a `GValue`. And I actually want here the actual value. My version did that