There is a 

ExpressionWalker walker(..)
expression_walker::walk<const Expression>(expr, &walker); 

pattern used in gen_expression.cpp to iterate over a tree of Expression objects using a visitor pattern.

Purpose of this PoC is to quickly assess whether the walker is suitable for evaluating the expression (i.e. move the ExpressionXYZ::evalute() body into the visit(ExpressionXYZ* node) method), especially to check:

1) every expression ends up evaluating all its children before computing its value

2) performances (compared with a Google Benchmark unit test) of a large operation (e.g. and Add operation with 5000 numerical values) are comparable with the current evaluate() approach