diff --git a/lib/fuzz/src/path_filter.ml b/lib/fuzz/src/path_filter.ml
index 61e5e252..c6fba971 100644
--- a/lib/fuzz/src/path_filter.ml
+++ b/lib/fuzz/src/path_filter.ml
@@ -125,13 +125,12 @@ let add_if (x : t) ~(when_ : bool) ~(add : t) : t =
   if when_ then x + add else x
 
 module Anchor_check = struct
-  type t = {is_nested: bool; pos: int; len: int; block_len: int}
+  type t = {is_nested: bool; span: Utils.My_list.Span.t; block_len: int}
   [@@deriving sexp]
 
   let of_path (path : Path.Stms.t) ~(block_len : int) : t =
     Path.Stms.
-      { pos= path.@(index)
-      ; len= len path
+      { span= {pos= path.@(index); len= len path}
       ; is_nested= is_nested path
       ; block_len }
 end
@@ -140,8 +139,7 @@ let is_anchored (anc : Path_meta.Anchor.t) ~(check : Anchor_check.t) : bool =
   check.is_nested
   || Path_meta.Anchor.(
        incl_opt ~includes:anc
-         (of_dimensions ~index:check.pos ~len:check.len
-            ~block_len:check.block_len))
+         (of_dimensions ~span:check.span ~block_len:check.block_len))
 
 let check_anchor (anc : Path_meta.Anchor.t) ~(path : Path.Stms.t)
     ~(block_len : int) : unit Or_error.t =
diff --git a/lib/fuzz/src/path_meta.ml b/lib/fuzz/src/path_meta.ml
index 9bc54e9e..a338cfd2 100644
--- a/lib/fuzz/src/path_meta.ml
+++ b/lib/fuzz/src/path_meta.ml
@@ -74,10 +74,12 @@ module Anchor = struct
   let incl_opt ?(includes : t option) (x : t option) : bool =
     [%equal: t option] x (merge_opt x includes)
 
-  let of_dimensions ~(index : int) ~(len : int) ~(block_len : int) : t option
-      =
-    let top = Option.some_if (0 = index) Top in
-    let bot = Option.some_if (block_len <= index + len) Bottom in
+  let of_dimensions ~(span : Utils.My_list.Span.t) ~(block_len : int) :
+      t option =
+    let top = Option.some_if (0 = span.pos) Top in
+    let bot =
+      Option.some_if (block_len <= Utils.My_list.Span.end_pos span) Bottom
+    in
     merge_opt top bot
 end
 
diff --git a/lib/fuzz/src/path_meta.mli b/lib/fuzz/src/path_meta.mli
index 77acce7c..8c2eb4da 100644
--- a/lib/fuzz/src/path_meta.mli
+++ b/lib/fuzz/src/path_meta.mli
@@ -83,9 +83,9 @@ module Anchor : sig
   (** [merge_opt l r] merges two optional anchors, according to the lattice
       formed by [incl_opt]. *)
 
-  val of_dimensions : index:int -> len:int -> block_len:int -> t option
-  (** [of_dimensions ~index ~len ~block_len] determines the anchor, if any,
-      from the given dimensions. *)
+  val of_dimensions : span:Utils.My_list.Span.t -> block_len:int -> t option
+  (** [of_dimensions ~span ~block_len] determines the anchor, if any, from
+      the given dimensions. *)
 end
 
 (** {1 Metadata structures} *)
diff --git a/lib/utils/src/my_list.ml b/lib/utils/src/my_list.ml
index 15f80109..9b9bcdee 100644
--- a/lib/utils/src/my_list.ml
+++ b/lib/utils/src/my_list.ml
@@ -12,7 +12,9 @@
 open Base
 
 module Span = struct
-  type t = {pos: int; len: int}
+  type t = {pos: int; len: int} [@@deriving sexp]
+
+  let end_pos ({pos; len} : t) : int = pos + len
 end
 
 let find_at_most_one (type a b) ?(item_name : string = "item")
diff --git a/lib/utils/src/my_list.mli b/lib/utils/src/my_list.mli
index 148be340..db76a4d0 100644
--- a/lib/utils/src/my_list.mli
+++ b/lib/utils/src/my_list.mli
@@ -17,7 +17,10 @@ open Base
 
 module Span : sig
   (** A span, containing a position and length. *)
-  type t = {pos: int; len: int}
+  type t = {pos: int; len: int} [@@deriving sexp]
+
+  val end_pos : t -> int
+  (** [end_pos] is the end position of the span: [pos + len]. *)
 end
 
 (** {1 Finding and transforming specific amounts of items} *)