op_new()
instantiates a newop
node.op_type()
checks if the node is aop
node.op_<child>_get()
returns the<child>
node ofop
.op_<child>_set()
sets the new<child>
and deletes the old<child>
.op_<child>_rep()
sets the new<child>
and returns the old<child>
.