Preserve mutability in "select"

M2/Macaulay2/d/actors4.d

@@ -156,7 +156,7 @@ select(e:Expr,f:Expr):Expr := (
 	  c := select(b.v,f);
 	  when c
 	  is Error do c
-	  is v:Sequence do list(b.Class,v)
+	  is v:Sequence do list(b.Class,v,b.Mutable)
 	  else e			  -- shouldn't happen
 	  )
      is n:ZZcell do (
@@ -221,7 +221,7 @@ select(n:Expr,e:Expr,f:Expr,g:Expr,h:Expr):Expr := (
 	  c := select(toInt(n),b.v,f);
 	  when c
 	  is Error do c
-	  is v:Sequence do list(b.Class,v)
+	  is v:Sequence do list(b.Class,v,b.Mutable)
 	  else e			  -- shouldn't happen
 	  )
      else WrongArg(1+1,"a list")

M2/Macaulay2/tests/normal/mutable-lists.m2

@@ -0,0 +1,12 @@
+-- select w/ mutable lists
+M = new MutableList from 1..10
+M' = select(M, even)
+assert Equation(#M', 5)
+for i to 4 do assert Equation(M'#i, 2*(i + 1))
+M'#0 = 5
+assert Equation(M'#0, 5)
+M'' = select(3, M, even)
+assert Equation(#M'',3)
+for i to 2 do assert Equation(M''#i, 2*(i + 1))
+M''#0 = 5
+assert Equation(M''#0, 5)