diff --git a/th01/hardware/grpinv32.hpp b/th01/hardware/grpinv32.hpp
index 94c332c4..e22e543d 100644
--- a/th01/hardware/grpinv32.hpp
+++ b/th01/hardware/grpinv32.hpp
@@ -36,12 +36,44 @@ inline long entrance_ring_radius(pixel_t base, unsigned int square_i) {
 	for( \
 		square_i = (1 * ENTRANCE_RING_SQUARES); \
 		square_i < ((1 + ENTRANCE_RING_STACK) * ENTRANCE_RING_SQUARES); \
-		i++ \
+		square_i++ \
 	) { \
 		invert_lhs entrance_ring_invert(square_i, radius_base); \
 	} \
 }
 
-#define entrance_rings_done(squares_offscreen) \
-	(squares_offscreen >= (ENTRANCE_RING_STACK * ENTRANCE_RING_SQUARES))
+// Well, well, we'd *really* like call sites to use this macro within an `if`
+// statement. That makes it looks as if this macro is actually a function that
+// returns whether the animation is done or not. Unfortunately, the completion
+// condition is itself nested within a conditional branch. This only leaves
+// ugly workarounds:
+//
+// 1) Forcing calling sites to close a block they didn't open
+// 2) A fake lambda function in another macro parameter
+// 3) Using `goto` and jumping to a label (`entrance_rings_still_active`) that
+//    we force the call site to define
+//
+// Option 3) doesn't introduce weird syntax and thus feels like the most
+// natural choice.
+#define entrance_rings_update_and_render( \
+	radius_base, i, squares_offscreen, frame, radius_base_initial, frame_first \
+) \
+	frame == frame_first) { \
+		radius_base = radius_base_initial; \
+		goto entrance_rings_still_active; \
+	} else if(frame == (frame_first + 1)) { \
+		entrance_rings_invert(i, {}, radius_base); \
+		goto entrance_rings_still_active; \
+	} else if((frame > (frame_first + 1)) && ((frame % 4) == 0)) { \
+		/* "Un-invert" the previous frame */ \
+		entrance_rings_invert(i, {}, radius_base); \
+		\
+		radius_base += 16; \
+		squares_offscreen = 0; \
+		\
+		entrance_rings_invert(i, squares_offscreen +=, radius_base); \
+	} else { \
+		goto entrance_rings_still_active; \
+	} \
+	if(squares_offscreen >= (ENTRANCE_RING_STACK * ENTRANCE_RING_SQUARES)
 /// ------------------------------------------------
diff --git a/th01/main/boss/b20m.cpp b/th01/main/boss/b20m.cpp
index e72d075d..693da6fa 100644
--- a/th01/main/boss/b20m.cpp
+++ b/th01/main/boss/b20m.cpp
@@ -2648,39 +2648,32 @@ void sariel_main(void)
 		random_seed = frame_rand;
 
 		while(1) {
-			boss_phase_frame++;
-			if(boss_phase_frame == 1) {
-				entrance_ring_radius_base = 16;
-			} else if(boss_phase_frame == 2) {
-				entrance_rings_invert(i, {}, entrance_ring_radius_base);
-			} else if((boss_phase_frame > 2) && ((boss_phase_frame % 4) == 0)) {
-				// "Un-invert" the previous frame
-				entrance_rings_invert(i, {}, entrance_ring_radius_base);
+			#define frame_half	boss_phase_frame
 
-				entrance_ring_radius_base += 16;
-				unsigned int squares_offscreen = 0;
+			unsigned int tmp;
 
-				entrance_rings_invert(
-					i, squares_offscreen +=, entrance_ring_radius_base
-				);
-				if(entrance_rings_done(squares_offscreen)) {
-					boss_phase = 1;
-					phase.pattern_cur = 0;
-					phase.ax.patterns_done = 0;
-					phase.patterns_until_next = ((rand() % 6) + 1);
-					boss_phase_frame = 0;
-					initial_hp_rendered = 0;
-					boss_palette_show(); // Unnecessary.
-					ent_shield.pos_cur_set(SHIELD_LEFT, SHIELD_TOP);
-					wand_lowered_snap();
-					wand_render_raise_both(true);
-					birds_reset();
-					return;
-				}
+			frame_half++;
+			if(entrance_rings_update_and_render(
+				entrance_ring_radius_base, i, tmp, frame_half, 16, 1
+			)) {
+				boss_phase = 1;
+				phase.pattern_cur = 0;
+				phase.ax.patterns_done = 0;
+				phase.patterns_until_next = ((rand() % 6) + 1);
+				boss_phase_frame = 0;
+				initial_hp_rendered = 0;
+				boss_palette_show(); // Unnecessary.
+				ent_shield.pos_cur_set(SHIELD_LEFT, SHIELD_TOP);
+				wand_lowered_snap();
+				wand_render_raise_both(true);
+				birds_reset();
+				break;
 			}
-			if(boss_phase_frame % 2) {
+entrance_rings_still_active:
+			if(frame_half % 2) { // That's why we've renamed the variable
 				frame_delay(1);
 			}
+			#undef frame_half
 		}
 	} else if(boss_phase == 1) {
 		hud_hp_increment_render(initial_hp_rendered, boss_hp, boss_phase_frame);