Debug print
nextNode nextTree fix
Segfault fix

Explanation:
For code(using labels e1, e2) like this:

sample_clause
    :    sample_key block_key?
        LEFT_PAREN e1=expression (COMMA e2=expression)? RIGHT_PAREN
        seed_part?
        -> ^(sample_key block_key? ^(EXPR $e1) ^(EXPR $e2)? seed_part?)
    ;

ANTLR generates code like this:
	e1=expression();
	...
	if ( this->get_backtracking()==0 ) stream_expression.add(e1.tree);
	...
	...
        // AST REWRITE
        // elements: e1, e2, block_key, sample_key, seed_part
        // token labels:
        // rule labels: retval, e1, e2
        // token list labels:
        // rule list labels:
        // wildcard labels:
        if ( this->get_backtracking()==0 ) {
            retval.tree = std::move(root_0);
            RewriteRuleSubtreeStream<ImplTraits> stream_retval(get_psrstate()->get_treeAdaptor(), "rule retval",retval.tree); // retval
            RewriteRuleSubtreeStream<ImplTraits> stream_e1(get_psrstate()->get_treeAdaptor(), "rule e1",e1.tree); // rewrite alias
            RewriteRuleSubtreeStream<ImplTraits> stream_e2(get_psrstate()->get_treeAdaptor(), "rule e2",e2.tree); // rewrite alias
	...

The variable e1 of type expression_return is assigned into two different SubTreeStreams.
Since destructive copying is used(unique_ptr) the second assignment inserts NULL ptr into stream_e1.

A new variable of type TreeType* (pure pointer) is added. A value of this pointer is used ONLY when
inserting a tree into alias'es RewriteRuleSubtreeStream. When there are no aliases declared in the rule the "last pointer" remains unused.
When inserting TreeTypePtr into a stream value is moved. When inserting a pure pointer into the aliases stream the whole tree is deeply dupped
(assuming that the original tree was already moved into some other RewriteRuleSubtreeStream).

New result:
        if ( this->get_backtracking()==0 ) {
            retval.tree = std::move(root_0);
            RewriteRuleSubtreeStream<ImplTraits> stream_retval(get_psrstate()->get_treeAdaptor(), "rule retval",retval.tree); // retval
            RewriteRuleSubtreeStream<ImplTraits> stream_e1(get_psrstate()->get_treeAdaptor(), "rule e1",e1_last); // rewrite alias
            RewriteRuleSubtreeStream<ImplTraits> stream_e2(get_psrstate()->get_treeAdaptor(), "rule e2",e2_last); // rewrite alias

Note: we have also deal with this situation.

unpivot_in_elements
    :   (    column_name
        |    LEFT_PAREN column_name (COMMA column_name)* RIGHT_PAREN
        )
        (     as_key
         (    constant
         |    (LEFT_PAREN)=> LEFT_PAREN constant (COMMA constant)* RIGHT_PAREN
         )
        )?
        -> column_name+ ^(PIVOT_ALIAS constant+)?
    ;

The AST tree contains multiple instances of "column_name".
RewriteRuleSubtreeStream can not contain reference(pointer) into matched tree as it would get overwritten
in every "loop". It really needs to be moved.
memory leak fix