protected T defaultResult() {
		return null;
	}
Gets the default value returned by visitor methods. This value is
returned by the default implementations of
{@link #visitTerminal visitTerminal}, {@link #visitErrorNode visitErrorNode}.
The default implementation of {@link #visitChildren visitChildren}
initializes its aggregate result to this value.

<p>The base implementation returns {@code null}.</p>

@return The default value returned by visitor methods.
runtime/Java/src/org/antlr/v4/runtime/tree/AbstractParseTreeVisitor.java