beg_new()
instantiates a newbeg
node.beg_type()
checks if the node is abeg
node.beg_<child>_get()
returns the<child>
node ofbeg
.beg_<child>_set()
sets the new<child>
and deletes the old<child>
.beg_<child>_rep()
sets the new<child>
and returns the old<child>
.