private InMemoryFunctionTag getImmutableTag() throws UsrException {

		FunctionTagTable table = getSourceTable();
		FunctionTagTableModel model = (FunctionTagTableModel) table.getModel();
		Optional<FunctionTagRowObject> optional = model.getModelData()
				.stream()
				.filter(row -> row.isImmutable())
				.findAny();
		assertTrue("No Immutable tags found", optional.isPresent());
		FunctionTag foundTag = optional.get().getTag();
		return (InMemoryFunctionTag) foundTag;
	}
Returns an immutable tag (the first one found) from the source panel, if one exists. 

@return an immutable tag, or null if not found
@throws UsrException if there's an error retrieving tags in the source panel
Ghidra/Features/Base/src/test.slow/java/ghidra/app/plugin/core/function/tags/FunctionTagPluginTest.java