code: @Override
    protected void visitFunctionNode(FunctionNode node) {
  
      // Cannot simplify nonplugin functions.
      // TODO(brndn): we can actually simplify checkNotNull.
      if (node.getSoyFunction() instanceof BuiltinFunction) {
        return;
      }
      if (node.getSoyFunction() instanceof LoggingFunction) {
        return;
      }
      // Default to fallback implementation.
      visitExprNode(node);
    }
docs: Implementations for functions.