  We address a declarative construction of abstract syntax trees with Parsing
Expression Grammars. AST operators (constructor, connector, and tagging) are
newly defined to specify flexible AST constructions. A new challenge coming
with PEGs is the consistency management of ASTs in backtracking and packrat
parsing. We make the transaction AST machine in order to perform AST operations
in the context of the speculative parsing of PEGs. All the consistency control
is automated by the analysis of AST operators. The proposed approach is
implemented in the Nez parser, written in Java. The performance study shows
that the transactional AST machine requires 25\% approximately more time in
CSV, XML, and C grammars.
