fixed non-optimal guard with nested groups

nested groups produce non-optimal guard (redundant parentheses)
-> group guard now adds elements of groups instead of the group itself

added test case

org.eclipse.xtext.tests/src/org/eclipse/xtext/xtext/generator/hoisting/HoistingProcessorTest.java

@@ -225,6 +225,23 @@ public class HoistingProcessorTest extends AbstractXtextTests {
 		assertTrue(guard.hasTerminal());
 	}
 
+	@Test
+	public void testGroupInGroup_expectMinimalParentheses() throws Exception {
+		// @formatter:off
+		String model =
+			MODEL_PREAMBLE +
+			"S: $$ p0 $$?=> ($$ p1 $$?=> ($$ p2 $$?=> 'a')); ";
+		// @formatter:off
+		XtextResource resource = getResourceFromString(model);
+		Grammar grammar = ((Grammar) resource.getContents().get(0));
+		AbstractRule rule = getRule(grammar, "S");
+		
+		HoistingGuard guard = hoistingProcessor.findHoistingGuard(rule.getAlternatives());
+		assertFalse(guard.isTrivial());
+		assertTrue(guard.hasTerminal());
+		assertEquals("((p0) && (p1) && (p2))", guard.render());
+	}
+	
 	@Test
 	public void testCardinalityPlusPredicate_expectPredicateAfterGroupNotInGuard() throws Exception {
 		// @formatter:off

org.eclipse.xtext.xtext.generator/src/org/eclipse/xtext/xtext/generator/parser/antlr/hoisting/guards/GroupGuard.java

@@ -31,8 +31,15 @@ public class GroupGuard implements HoistingGuard {
 	}
 	
 	public void add(Guard guard) {
-		if (!guard.isTrivial())
+		if (guard.isTrivial()) {
+			return;
+		}
+		
+		if (guard instanceof GroupGuard) {
+			((GroupGuard) guard).elementGuards.forEach(this::add);
+		} else {
 			elementGuards.add(guard);
+		}
 	}
 	
 	public void setHasTerminal() {