The node will have one child. It owns it.
| NodeUn (UnOper* op, Node* node, unsigned line) | NodeUn | 
Caller must not delete / modify pointers.
Parameters:
| op | unary operation associated with node. | 
| node | child of node -- argument of operation. | 
| line | line of configuration where node was created. | 
| ~NodeUn () | ~NodeUn | 
| DEFINE_VISITABLE () | DEFINE_VISITABLE | 
| const UnOper&  op () | op | 
[const]
Returns: unary operation associated with node.
| Node&  node () | node | 
[const]
Returns: argument of unary operation.