patterns: inject into <bind …>

src/syndicate/patterns.nim

@@ -221,16 +221,22 @@ proc inject*(pat: Pattern; bindings: openArray[(int, Pattern)]): Pattern =
   proc inject(pat: Pattern; bindings: openArray[(int, Pattern)]; offset: var int): Pattern =
     case pat.orKind
     of PatternKind.DDiscard:
-      var replaced = false
-      for (i, injection) in bindings:
-        if i == offset:
+      result = pat
+      for (off, injection) in bindings:
+        if off == offset:
           result = injection
-          replaced = true
           break
-      if not replaced:
-        result = drop()
       inc offset
-    of PatternKind.DBind, PatternKind.DLit:
+    of PatternKind.DBind:
+      let bindOff = offset
+      result = pat
+      result.dbind.pattern = inject(pat.dbind.pattern, bindings, offset)
+      if result.orKind == PatternKind.DBind:
+        for (off, injection) in bindings:
+          if (off == bindOff) and (result.dbind.pattern == injection):
+            result = result.dbind.pattern
+            break # promote the injected pattern over the bind
+    of PatternKind.DLit:
       result = pat
     of PatternKind.DCompound:
       result = pat