diff --git a/AST.ml b/AST.ml
index f3b6a3ef7..a57d2e47d 100644
--- a/AST.ml
+++ b/AST.ml
@@ -43,6 +43,7 @@ type kwd_and        = Region.t
 type kwd_begin      = Region.t
 type kwd_case       = Region.t
 type kwd_const      = Region.t
+type kwd_contains   = Region.t
 type kwd_down       = Region.t
 type kwd_else       = Region.t
 type kwd_end        = Region.t
@@ -394,7 +395,7 @@ and fail_instr = {
 
 and conditional = {
   kwd_if     : kwd_if;
-  test       : expr;
+  test       : test_expr;
   kwd_then   : kwd_then;
   ifso       : instruction;
   terminator : semi option;
@@ -402,6 +403,16 @@ and conditional = {
   ifnot      : instruction
 }
 
+and test_expr =
+  GenExpr of expr
+| SetMem  of set_membership reg
+
+and set_membership = {
+  set          : expr;
+  kwd_contains : kwd_contains;
+  element      : expr
+}
+
 and case_instr = {
   kwd_case  : kwd_case;
   expr      : expr;
@@ -1041,13 +1052,23 @@ and print_conditional node =
   let {kwd_if; test; kwd_then; ifso; terminator;
        kwd_else; ifnot} = node in
   print_token       kwd_if "if";
-  print_expr        test;
+  print_test_expr   test;
   print_token       kwd_then "then";
   print_instruction ifso;
   print_terminator  terminator;
   print_token       kwd_else "else";
   print_instruction ifnot
 
+and print_test_expr = function
+  GenExpr e -> print_expr e
+| SetMem  m -> print_set_membership m
+
+and print_set_membership {value; _} =
+  let {set; kwd_contains; element} = value in
+  print_expr set;
+  print_token kwd_contains "contains";
+  print_expr element
+
 and print_case_instr (node : case_instr) =
   let {kwd_case; expr; kwd_of;
        lead_vbar; cases; kwd_end} = node in
diff --git a/AST.mli b/AST.mli
index b1bd5d57f..fbf7545d9 100644
--- a/AST.mli
+++ b/AST.mli
@@ -27,6 +27,7 @@ type kwd_and        = Region.t
 type kwd_begin      = Region.t
 type kwd_case       = Region.t
 type kwd_const      = Region.t
+type kwd_contains   = Region.t
 type kwd_down       = Region.t
 type kwd_else       = Region.t
 type kwd_end        = Region.t
@@ -378,7 +379,7 @@ and fail_instr = {
 
 and conditional = {
   kwd_if     : kwd_if;
-  test       : expr;
+  test       : test_expr;
   kwd_then   : kwd_then;
   ifso       : instruction;
   terminator : semi option;
@@ -386,6 +387,16 @@ and conditional = {
   ifnot      : instruction
 }
 
+and test_expr =
+  GenExpr of expr
+| SetMem  of set_membership reg
+
+and set_membership = {
+  set          : expr;
+  kwd_contains : kwd_contains;
+  element      : expr
+}
+
 and case_instr = {
   kwd_case  : kwd_case;
   expr      : expr;
diff --git a/LexToken.mli b/LexToken.mli
index 0f219f215..b051120ae 100644
--- a/LexToken.mli
+++ b/LexToken.mli
@@ -69,6 +69,7 @@ type t =
 | Begin      of Region.t  (* "begin"      *)
 | Case       of Region.t  (* "case"       *)
 | Const      of Region.t  (* "const"      *)
+| Contains   of Region.t  (* "contains"   *)
 | Down       of Region.t  (* "down"       *)
 | Else       of Region.t  (* "else"       *)
 | End        of Region.t  (* "end"        *)
diff --git a/LexToken.mll b/LexToken.mll
index b5f1de1f4..ee2d663e3 100644
--- a/LexToken.mll
+++ b/LexToken.mll
@@ -68,6 +68,7 @@ type t =
 | Begin      of Region.t  (* "begin"      *)
 | Case       of Region.t  (* "case"       *)
 | Const      of Region.t  (* "const"      *)
+| Contains   of Region.t  (* "contains"   *)
 | Down       of Region.t  (* "down"       *)
 | Else       of Region.t  (* "else"       *)
 | End        of Region.t  (* "end"        *)
@@ -191,6 +192,7 @@ let proj_token = function
 | Begin      region -> region, "Begin"
 | Case       region -> region, "Case"
 | Const      region -> region, "Const"
+| Contains   region -> region, "Contains"
 | Down       region -> region, "Down"
 | Else       region -> region, "Else"
 | End        region -> region, "End"
@@ -279,6 +281,7 @@ let to_lexeme = function
 | Begin      _ -> "begin"
 | Case       _ -> "case"
 | Const      _ -> "const"
+| Contains   _ -> "contains"
 | Down       _ -> "down"
 | Fail       _ -> "fail"
 | If         _ -> "if"
@@ -337,6 +340,7 @@ let keywords = [
   (fun reg -> Begin      reg);
   (fun reg -> Case       reg);
   (fun reg -> Const      reg);
+  (fun reg -> Contains   reg);
   (fun reg -> Down       reg);
   (fun reg -> Fail       reg);
   (fun reg -> If         reg);
@@ -563,6 +567,7 @@ let is_kwd = function
 | Begin      _
 | Case       _
 | Const      _
+| Contains   _
 | Down       _
 | Fail       _
 | If         _
diff --git a/ParToken.mly b/ParToken.mly
index 9d5efd7c1..1487ff491 100644
--- a/ParToken.mly
+++ b/ParToken.mly
@@ -46,6 +46,7 @@
 %token <Region.t> Begin       (* "begin"      *)
 %token <Region.t> Case        (* "case"       *)
 %token <Region.t> Const       (* "const"      *)
+%token <Region.t> Contains    (* "contains"   *)
 %token <Region.t> Down        (* "down"       *)
 %token <Region.t> Fail        (* "fail"       *)
 %token <Region.t> From        (* "from"       *)
diff --git a/Parser.mly b/Parser.mly
index e57c6c4bb..3d3cbddfe 100644
--- a/Parser.mly
+++ b/Parser.mly
@@ -581,7 +581,7 @@ proc_call:
   fun_call { $1 }
 
 conditional:
-  If expr Then instruction option(SEMI) Else instruction {
+  If test_expr Then instruction option(SEMI) Else instruction {
     let region = cover $1 (instr_to_region $7) in
     let value = {
       kwd_if     = $1;
@@ -594,6 +594,22 @@ conditional:
     in {region; value}
   }
 
+test_expr:
+  expr           { GenExpr $1 }
+| set_membership {  SetMem $1 }
+
+set_membership:
+  expr Contains expr {
+    let start  = expr_to_region $1
+    and stop   = expr_to_region $3 in
+    let region = cover start stop in
+    let value  = {
+      set          = $1;
+      kwd_contains = $2;
+      element      = $3}
+    in {region; value}
+  }
+
 case_instr:
   Case expr Of option(VBAR) cases End {
     let region = cover $1 $6 in