presto-parser/src/main/java/io/prestosql/sql/tree/DefaultTraversalVisitor.java/DefaultTraversalVisitor/visitWhenClause
class DefaultTraversalVisitor:    @Override
    protected R visitWhenClause(WhenClause node, C context)
    {
        process(node.getOperand(), context);
        process(node.getResult(), context);

        return null;
    }