Issue 4205: Improve part combiner's rest analysis

input/regression/part-combine-silence.ly

Index: input/regression/part-combine-silence.ly
diff --git a/input/regression/part-combine-silence.ly b/input/regression/part-combine-silence.ly
new file mode 100644
index 0000000000000000000000000000000000000000..796242b80f47ba2a0b9d0459eab9b2993feb6b10
--- /dev/null
+++ b/input/regression/part-combine-silence.ly
@@ -0,0 +1,31 @@
+\version "2.19.16"
+
+\header {
+  texidoc = "Rests must begin and end simultaneously to be merged into the shared voice."
+}
+
+% rests of different durations beginning simultaneously, followed by
+% unisilence
+
+\score {
+  \partcombine
+    \relative f' { r4    r2 r8 r8 | r1 }
+    \relative f' { r8 r8 r2 r4    | r1 }
+}
+
+% rests of different durations beginning simultaneously, followed by
+% solo then a2.
+
+\score {
+  \partcombine
+    \relative f' { r4   f2. | r8 f e2. }
+    \relative f' { r8 d f2. | r4   e2. }
+}
+
+% mmrest and rest of different durations beginning simultaneously
+
+\score {
+  \partcombine
+    \relative f' { r4 f2. | R1 }
+    \relative f' { R1     | r4 d2. }
+}