| NodeAssign (const string& varid, Node* rvalue, unsigned line) | NodeAssign | 
Caller must not delete / modify pointer.
Parameters:
| varid | the name of the variable being assigned to. | 
| rvalue | the expression being assigned to the variable. | 
| line | line of configuration where node was created. | 
| ~NodeAssign () | ~NodeAssign | 
| DEFINE_VISITABLE () | DEFINE_VISITABLE | 
| const string&  varid () | varid | 
[const]
Returns: name of variable being assigned to.
| Node&  rvalue () | rvalue | 
[const]
Returns: argument of assignment.