diff --git a/tapl/README.md b/tapl/README.md
index dc86988..0b1a435 100644
--- a/tapl/README.md
+++ b/tapl/README.md
@@ -14,5 +14,6 @@ A file extends its immediate parent file.
      - stlc+sub.rkt
        - stlc+reco+sub.rkt (also pull in tup from stlc+reco+var.rkt)
      - sysf.rkt
+       - fsub.rkt
        - fomega.rkt
        - fomega2.rkt
\ No newline at end of file
diff --git a/tapl/fsub.rkt b/tapl/fsub.rkt
new file mode 100644
index 0000000..8d4ca01
--- /dev/null
+++ b/tapl/fsub.rkt
@@ -0,0 +1,12 @@
+#lang racket/base
+(require "typecheck.rkt")
+(require (except-in "sysf.rkt" #%app)
+         (prefix-in sysf: (only-in "sysf.rkt" #%app)))
+(provide (rename-out [sysf:#%app #%app]))
+(provide (except-out (all-from-out "sysf.rkt") sysf:#%app))
+ 
+;; System F<:
+;; Types:
+;; - types from sysf.rkt
+;; Terms:
+;; - terms from sysf.rkt
diff --git a/tapl/tests/fsub-tests.rkt b/tapl/tests/fsub-tests.rkt
new file mode 100644
index 0000000..19d47ca
--- /dev/null
+++ b/tapl/tests/fsub-tests.rkt
@@ -0,0 +1,70 @@
+#lang s-exp "../fsub.rkt"
+(require "rackunit-typechecking.rkt")
+
+
+;; old sysf tests -------------------------------------------------------------
+(check-type (Λ (X) (λ ([x : X]) x)) : (∀ (X) (→ X X)))
+
+(check-type (Λ (X) (λ ([t : X] [f : X]) t)) : (∀ (X) (→ X X X))) ; true
+(check-type (Λ (X) (λ ([t : X] [f : X]) f)) : (∀ (X) (→ X X X))) ; false
+(check-type (Λ (X) (λ ([t : X] [f : X]) f)) : (∀ (Y) (→ Y Y Y))) ; false, alpha equiv
+
+(check-type (Λ (t1) (Λ (t2) (λ ([x : t1]) (λ ([y : t2]) y))))
+            : (∀ (t1) (∀ (t2) (→ t1 (→ t2 t2)))))
+
+(check-type (Λ (t1) (Λ (t2) (λ ([x : t1]) (λ ([y : t2]) y))))
+            : (∀ (t3) (∀ (t4) (→ t3 (→ t4 t4)))))
+
+(check-not-type (Λ (t1) (Λ (t2) (λ ([x : t1]) (λ ([y : t2]) y))))
+            : (∀ (t4) (∀ (t3) (→ t3 (→ t4 t4)))))
+
+(check-type (inst (Λ (t) (λ ([x : t]) x)) Int) : (→ Int Int))
+(check-type (inst (Λ (t) 1) (→ Int Int)) : Int)
+; first inst should be discarded
+(check-type (inst (inst (Λ (t) (Λ (t) (λ ([x : t]) x))) (→ Int Int)) Int) : (→ Int Int))
+; second inst is discarded
+(check-type (inst (inst (Λ (t1) (Λ (t2) (λ ([x : t1]) x))) Int) (→ Int Int)) : (→ Int Int))
+
+;;; polymorphic arguments
+(check-type (Λ (t) (λ ([x : t]) x)) : (∀ (t) (→ t t)))
+(check-type (Λ (t) (λ ([x : t]) x)) : (∀ (s) (→ s s)))
+(check-type (Λ (s) (Λ (t) (λ ([x : t]) x))) : (∀ (s) (∀ (t) (→ t t))))
+(check-type (Λ (s) (Λ (t) (λ ([x : t]) x))) : (∀ (r) (∀ (t) (→ t t))))
+(check-type (Λ (s) (Λ (t) (λ ([x : t]) x))) : (∀ (r) (∀ (s) (→ s s))))
+(check-type (Λ (s) (Λ (t) (λ ([x : t]) x))) : (∀ (r) (∀ (u) (→ u u))))
+(check-type (λ ([x : (∀ (t) (→ t t))]) x) : (→ (∀ (s) (→ s s)) (∀ (u) (→ u u))))
+(typecheck-fail ((λ ([x : (∀ (t) (→ t t))]) x) (λ ([x : Int]) x)))
+(typecheck-fail ((λ ([x : (∀ (t) (→ t t))]) x) 1))
+(check-type ((λ ([x : (∀ (t) (→ t t))]) x) (Λ (s) (λ ([y : s]) y))) : (∀ (u) (→ u u)))
+(check-type
+ (inst ((λ ([x : (∀ (t) (→ t t))]) x) (Λ (s) (λ ([y : s]) y))) Int) : (→ Int Int))
+(check-type
+ ((inst ((λ ([x : (∀ (t) (→ t t))]) x) (Λ (s) (λ ([y : s]) y))) Int) 10)
+ : Int ⇒ 10)
+(check-type (λ ([x : (∀ (t) (→ t t))]) (inst x Int)) : (→ (∀ (t) (→ t t)) (→ Int Int)))
+(check-type (λ ([x : (∀ (t) (→ t t))]) ((inst x Int) 10)) : (→ (∀ (t) (→ t t)) Int))
+(check-type ((λ ([x : (∀ (t) (→ t t))]) ((inst x Int) 10))
+             (Λ (s) (λ ([y : s]) y)))
+             : Int ⇒ 10)
+
+
+;;; previous tests -------------------------------------------------------------
+(check-type 1 : Int)
+(check-not-type 1 : (→ Int Int))
+(typecheck-fail "one") ; unsupported literal
+(typecheck-fail #f) ; unsupported literal
+(check-type (λ ([x : Int] [y : Int]) x) : (→ Int Int Int))
+(check-not-type (λ ([x : Int]) x) : Int)
+(check-type (λ ([x : Int]) x) : (→ Int Int))
+(check-type (λ ([f : (→ Int Int)]) 1) : (→ (→ Int Int) Int))
+(check-type ((λ ([x : Int]) x) 1) : Int ⇒ 1)
+(typecheck-fail ((λ ([x : Bool]) x) 1)) ; Bool is not valid type
+(typecheck-fail (λ ([x : Bool]) x)) ; Bool is not valid type
+(typecheck-fail (λ ([f : Int]) (f 1 2))) ; applying f with non-fn type
+(check-type (λ ([f : (→ Int Int Int)] [x : Int] [y : Int]) (f x y))
+            : (→ (→ Int Int Int) Int Int Int))
+(check-type ((λ ([f : (→ Int Int Int)] [x : Int] [y : Int]) (f x y)) + 1 2) : Int ⇒ 3)
+(typecheck-fail (+ 1 (λ ([x : Int]) x))) ; adding non-Int
+(typecheck-fail (λ ([x : (→ Int Int)]) (+ x x))) ; x should be Int
+(typecheck-fail ((λ ([x : Int] [y : Int]) y) 1)) ; wrong number of args
+(check-type ((λ ([x : Int]) (+ x x)) 10) : Int ⇒ 20)