diff --git a/_CoqProject b/_CoqProject
index 0932b31a0..aa7fd552a 100644
--- a/_CoqProject
+++ b/_CoqProject
@@ -119,5 +119,6 @@ theories/kernel.v
 theories/pi_irrational.v
 theories/gauss_integral.v
 theories/showcase/summability.v
+theories/showcase/sorgenfreyline.v
 analysis_stdlib/Rstruct_topology.v
 analysis_stdlib/showcase/uniform_bigO.v
diff --git a/theories/Make b/theories/Make
index ab6dede6e..3e1dbff6f 100644
--- a/theories/Make
+++ b/theories/Make
@@ -85,3 +85,4 @@ pi_irrational.v
 gauss_integral.v
 all_analysis.v
 showcase/summability.v
+showcase/sorgenfreyline.v
diff --git a/theories/showcase/sorgenfreyline.v b/theories/showcase/sorgenfreyline.v
new file mode 100644
index 000000000..095e87da1
--- /dev/null
+++ b/theories/showcase/sorgenfreyline.v
@@ -0,0 +1,263 @@
+From HB Require Import structures.
+From mathcomp Require Import all_ssreflect all_algebra all_fingroup.
+From mathcomp Require Import wochoice contra.
+From mathcomp Require Import boolp classical_sets set_interval.
+From mathcomp Require Import topology_structure separation_axioms connected.
+From mathcomp Require Import reals.
+
+Set Implicit Arguments.
+Unset Strict Implicit.
+Unset Printing Implicit Defensive.
+
+Import GRing.Theory Num.Theory Order.Theory.
+Local Open Scope ring_scope.
+Import boolp.
+
+Import classical_sets.
+Local Open Scope classical_set_scope.
+
+Section general_topology.
+Context {T : topologicalType}.
+
+Section extra.
+
+(* TODO: better name? *)
+Lemma nbhs_cobind (x y : T) :
+  (forall A, nbhs x A -> A y) -> forall A, nbhs x A -> nbhs y A.
+Proof.
+move=> + A /[1!(nbhsE x)] -[] U oU UA => /(_ U (open_nbhs_nbhs oU)) Uy.
+by rewrite nbhsE; exists U=> //; split; case: oU.
+Qed.
+
+Lemma all_and2P (P Q : T -> Prop):
+  (forall A, P A /\ Q A) <-> ((forall A, P A) /\ (forall A, Q A)).
+Proof. by firstorder. Qed.
+
+Lemma all_and2E (P Q : T -> Prop):
+  (forall A, P A /\ Q A) = ((forall A, P A) /\ (forall A, Q A)).
+Proof. exact/propeqP/all_and2P. Qed.
+
+End extra.
+
+Lemma kolmogorov_inj_nbhsP : kolmogorov_space T <-> injective (@nbhs _ T).
+Proof.
+split => i x y.
+  apply: contraPP => /eqP /i [] X [] [] /[swap] h1 /[swap] xy;
+  by rewrite inE in h1; rewrite (xy, esym xy) inE => /nbhs_singleton /h1.
+apply: contraNP => /asboolPn /forallp_asboolPn H.
+apply/eqP/i/predeqP => A.
+wlog: x y H / nbhs x A.
+  split => ?; first by rewrite -(H0 x).
+  by rewrite -(H0 y) => // B; rewrite boolp.orC.
+split=> //.
+rewrite nbhsE => -[] B /[dup] /open_nbhs_nbhs nbhsB [] opB Bx BA.
+move: (H B); rewrite not_orE !inE !not_andE -!implyE !notK.
+by case => /(_ nbhsB) By _; apply/(filterS BA) /open_nbhs_nbhs.
+Qed.
+
+End general_topology.
+
+Section Sorgenfrey_line.
+Variable R : realType.
+
+Let I := (R^o * R^o)%type.
+Let D := [set : I].
+Let b (d : I) := `[ d.1, d.2 [%classic.
+Let b_cover : \bigcup_(i in D) b i = setT. 
+Proof.
+apply/seteqP; split  => // x _ .
+exists (x, x+1)%R => //=.
+by rewrite /b /=  in_itv /= lexx /= ltrDl.
+Qed.
+Let b_join i j t : D i -> D j -> b i t -> b j t ->
+    exists k, [/\ D k, b k t & b k `<=` b i `&` b j].
+Proof.
+move=> Di Dj bit bjt.
+exists (Order.max i.1 j.1, Order.min i.2 j.2)%R.
+rewrite /D /=.
+rewrite /b /= in_itv.
+split; trivial.
+  rewrite /=.
+  rewrite comparable_ge_max; last first.
+    rewrite /Order.comparable.
+    apply:real_leVge; apply:num_real; apply:num_real.
+  rewrite comparable_lt_min; last first.
+    apply:real_leVge; apply:num_real; apply:num_real.
+  (*rewrite Bool.andb_assoc.*)
+  have:( t \in `[ i.1, i.2 [%R ).
+    by rewrite in_itv /=; apply bit.
+  have:( t \in `[ j.1, j.2 [%R ).
+    by rewrite in_itv /=; apply bjt.
+  Check(t \in `[j.1, j.2[%R).
+  rewrite !in_itv /=.
+  case/andP => -> ->.
+  by case/andP => -> ->.
+rewrite /=.
+move=> x.
+rewrite /= !in_itv /=.
+rewrite comparable_ge_max; last first.
+  apply:real_leVge; apply:num_real; apply:num_real.
+case/andP => /andP [] -> ->.
+rewrite comparable_lt_min; last first.
+  apply:real_leVge; apply:num_real; apply:num_real.
+by case/andP => -> ->.
+Qed.
+
+
+Fail Check R:topologicalType.
+Fail Check R^o : topologicalType.
+HB.about GRing.regular.
+(* prove I is pointed *)
+#[export, non_forgetful_inheritance]
+HB.instance Definition _ := @isPointed.Build R 0.
+HB.instance Definition Sorgenfrey_mixin := @isBaseTopological.Build R I D b b_cover b_join. 
+Definition sorgenfrey := HB.pack_for topologicalType _ Sorgenfrey_mixin.
+Check sorgenfrey : topologicalType.
+
+Lemma open_separated (T : topologicalType) (A B : set T) :
+  open A -> open B -> (A `&` B = set0 -> separated A B).
+Proof.
+move=> oA oB.
+apply:contraPP.
+rewrite /separated.
+rewrite not_andP.
+case.
+  contra.
+  rewrite disjoints_subset => AsubnB.
+  rewrite disjoints_subset.
+  move:(closure_subset AsubnB).
+  have:closed (~` B).
+    by apply:open_closedC.
+  move/closure_id => csB. 
+  by rewrite -csB.
+contra.
+rewrite setIC.
+rewrite disjoints_subset => BsubnA.
+rewrite setIC disjoints_subset.
+move:(closure_subset BsubnA).
+have:closed (~` A).
+  by apply:open_closedC.
+move/closure_id => csA.
+by rewrite -csA.
+Qed.
+
+Lemma separated_sub (T : topologicalType)(A B C : set T) :
+  separated A B -> C `<=` B -> separated A C.
+Proof.
+rewrite /separated.
+case.
+move=> cAB0 AcB0. 
+move=> CB.
+split.
+  rewrite setIC disjoints_subset.
+  apply (subset_trans CB).
+  by rewrite -disjoints_subset setIC.
+rewrite setIC disjoints_subset.
+apply (subset_trans (closure_subset CB)).
+by rewrite -disjoints_subset setIC.
+Qed.
+
+Check sorgenfrey.
+
+Lemma sorgenfrey_line_totally_disconnected : totally_disconnected [set: sorgenfrey].
+Proof.
+rewrite /totally_disconnected.
+move=> x Rx.
+(*rewrite /setT /set1.*)
+rewrite /connected_component.
+have:forall C : set sorgenfrey, C x -> connected C -> C = [set x].
+  move=> C Cx.
+  apply:contraPP.
+  move=> Cneqsetx.
+  have[y [yx Cy]]:(exists y, y != x /\ C y).
+    apply/not_existsP.
+    move:Cneqsetx.
+    contra.
+    move=> H.
+    apply:funext.
+    move=> y.
+    move:(H y).
+    rewrite not_andE.
+    move=>Hy.
+    rewrite propeqP /=.
+    split.
+      case:Hy => //.
+      move/negP.
+      rewrite negbK.
+      by move/eqP.
+    by move->.
+  wlog xy : x y yx Cx Cy {Rx Cneqsetx} / x < y.
+    move:(yx). 
+    rewrite neq_lt.
+    case/orP.
+      move=> ylx H.
+      apply:(H y x) => //.
+      by rewrite eq_sym.        
+    move=> xly H.
+    by apply:(H x y) => //.
+  rewrite connectedPn.
+  exists (fun b => if b then C `&` `]-oo, y[ else C `&` `[ y, +oo[).
+  split.
+  - case.
+      exists x.
+      by split.
+    exists y.
+    split => //=.
+    by rewrite in_itv /= lexx.
+  - rewrite -setIUr.
+    move:(setCitv `[y, y[%R ).
+    rewrite /=.
+    rewrite setUC => <-.
+    by rewrite set_itvco0 setC0 setIT.
+  apply: (separated_sub (B := `]-oo, y[)); last by apply: subIsetr.
+  rewrite separatedC. 
+  apply: (separated_sub (B := `[y, +oo[)); last by apply: subIsetr.
+  apply: open_separated. 
+  - have -> : `]-oo, y[ = \bigcup_( z in `]-oo, y[ ) `[z, y[.
+      apply/seteqP.
+      split => w Hw /=.
+        exists w => //=.
+        move: Hw.
+        by rewrite /= !in_itv /= lexx => ->.
+      by case: Hw => t /= /[!in_itv] /= _ /andP [].
+    apply: bigcup_open => w ? /=.
+    rewrite /open /=.
+    exists [set (w, y)] => //.
+    by rewrite bigcup_set1.
+  - have -> : `[y, +oo[ = \bigcup_( z in `[y, +oo[ ) `[y, z[.
+      apply/seteqP.
+      split => w Hw /=.
+        exists (w+1) => //=.
+          move: Hw.
+          rewrite /= !in_itv /=.
+          rewrite andbC /= andbC /=.
+          by apply/(ler_wpDr ler01).
+        rewrite in_itv /=.
+        apply/andP.
+        split.
+          move:Hw. 
+          by rewrite /= in_itv /= andbC.
+        apply:ltr_pwDr; last by [].
+        by apply: ltr01.
+      rewrite in_itv /= andbC /=.
+      by case: Hw => t /= /[!in_itv] /= _ /andP [].
+    apply: bigcup_open => w ? /=.
+    rewrite /open /=.
+    exists [set (y, w)] => //.
+    by rewrite bigcup_set1.
+  apply/seteqP.
+  split => w //=.
+  by rewrite !in_itv /= leNgt => -[] ->.
+move=> H.
+rewrite ( _ : [set C | _ ] = [set [set x]]). 
+  by rewrite bigcup_set1.
+apply/seteqP.
+split => C //=.
+  case=>*.
+  exact: H.
+move->.
+split => //.
+exact: connected1.
+Qed.
+
+End Sorgenfrey_line.