public String getHeaderFileName() {
		ST extST = getTemplates().getInstanceOf("headerFileExtension");
		if ( extST==null ) return null;
		String recognizerName = g.getRecognizerName();
		return recognizerName+extST.render();
	}
What is the name of the vocab file generated for this grammar?
 Returns null if no .tokens file should be generated.
tool/src/org/antlr/v4/codegen/CodeGenerator.java