more debug strings

src/testdata/sandbox.txt

@@ -0,0 +1,5 @@
+. a . b . c
+2a2  2b2 2c2
+
+. d . e . f
+2d2 2e2 2f2

src/tone.rs

@@ -84,21 +84,24 @@ impl Tone {
     }
 
     pub fn string(&self) -> String {
-        match self.i32() {
-            57|69 => "a",
-            58|70 => "a+",
-            59|71 => "b",
-            60|72 => "c",
-            61|73 => "c+",
-            62|74 => "d",
-            63|75 => "d+",
-            64|76 => "e",
-            65|77 => "f",
-            66|78 => "f+",
-            67|79 => "g",
-            68|80 => "g+",
+        let v = self.i32();
+        let modifier = if v > 0 && v < 57 { "-" } else if v >= 69 { "+" } else { "" };
+        modifier.to_string() + match v {
+            45|57|69 => "a",
+            46|58|70 => "a+",
+            47|59|71 => "b",
+            48|60|72 => "c",
+            49|61|73 => "c+",
+            50|62|74 => "d",
+            51|63|75 => "d+",
+            52|64|76 => "e",
+            53|65|77 => "f",
+            54|66|78 => "f+",
+            55|67|79 => "g",
+            56|68|80 => "g+",
+            0 => " ",
             _ => "?",
-        }.to_string()
+        }
     }
 }