This fixes a bug which can result in a value out of range error if the
cursor is at the beginning of a TextPort and the DeleteToStartOfLine
procedure is invoked.

This patch has been sent to SRC.

Index: m3/vbtkit/src/etext/TextPortClass.m3
--- TextPortClass.m3-	Mon Jan 30 14:34:34 1995
+++ TextPortClass.m3	Thu May  8 17:20:58 1997
@@ -271,8 +271,12 @@
     left := MTextUnit.StartOfLine (v.vtext.mtext, here);
   BEGIN
     IF here = left THEN
-      (* We're already at the start of line; delete one char. *)
-      RETURN v.replace (here - 1, here, "")
+      (* Already at the start of line; delete preceding newline if any. *)
+      IF here > 0 THEN
+        RETURN v.replace (here - 1, here, "")
+      ELSE
+        RETURN NotFound
+      END
     ELSE
       RETURN v.replace (left, here, "")
     END