s messes up our clean declarative grammars.
        If we care to keep our code clean, <code class="computeroutput"><span class="identifier">on_success</span></code>
        handlers are alternative callback hooks to client code that are executed
        by the parser after a successful parse without polluting the grammar. Like
        semantic actions, <code class="computeroutput"><span class="identifier">on_success</span></code>
        handlers have access to the AST, the iterators, and context. But, unl