diff --git a/ack.rkt b/ack.rkt
new file mode 100644
index 0000000..eccb418
--- /dev/null
+++ b/ack.rkt
@@ -0,0 +1,7 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname ack) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f () #f)))
+(define (ack n m)
+  (cond [(= 0 n) (+ m 1)]
+        [(= 0 m) (ack (- n 1) 1)]
+        [else (ack (- n 1) (ack n (- m 1)))]))
\ No newline at end of file
diff --git a/bst.rkt b/bst.rkt
index 9256ee6..266f6b9 100644
--- a/bst.rkt
+++ b/bst.rkt
@@ -108,4 +108,9 @@
     [(empty? node) empty]
     [(empty? (node-left node)) (node-right node)]
     [(empty? (node-right node)) (node-left node)]
-    [else (make-node (node-value (replacement node)) (sans-max-subtree (node-left node)) (node-right node))]))
\ No newline at end of file
+    [else (make-node (node-value (replacement node)) (sans-max-subtree (node-left node)) (node-right node))]))
+
+(check-equal? (delete empty) empty)
+(check-equal? (delete (make-node 5 empty empty)) empty)
+(check-equal? (make-node 5 empty empty) (make-node 5 empty empty))
+(check-equal? (delete (make-node 6 (make-node 2 empty empty) empty)) (make-node 2 empty empty))
\ No newline at end of file
diff --git a/fop-klausur.rkt b/fop-klausur.rkt
new file mode 100644
index 0000000..841c06c
--- /dev/null
+++ b/fop-klausur.rkt
@@ -0,0 +1,20 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname fop-klausur) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f () #f)))
+(define-struct abc (a b))
+
+(define (foo p)
+  (if (abc? p)
+      (if (list? (abc-b p))
+          (cons (abc-a p) (abc-b p))
+          empty)
+      empty))
+
+(define (bar lst n)
+  (cond
+    [(= 0 n) empty]
+    [(< (length lst) n) lst]
+    [else (cons (first lst) (bar (rest lst) (- n 1)))]))
+
+(define (foo-bar n)
+  (lambda (x) (bar x n)))
\ No newline at end of file
diff --git a/pascal-line.rkt b/pascal-line.rkt
new file mode 100644
index 0000000..c7e7b95
--- /dev/null
+++ b/pascal-line.rkt
@@ -0,0 +1,10 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname pascal-line) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f () #f)))
+(define (pascal-line n)
+  (cond [(= n 0) (list 1)]
+        [else (new-pascal-line (append (list 0) (pascal-line (- n 1)) (list 0)))]))
+
+(define (new-pascal-line l)
+  (cond [(< (length l) 2) empty]
+        [else (cons (+ (first l) (second l)) (new-pascal-line (rest l)))]))
\ No newline at end of file
diff --git a/substr.rkt b/substr.rkt
new file mode 100644
index 0000000..6eff8b3
--- /dev/null
+++ b/substr.rkt
@@ -0,0 +1,14 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname substr) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f () #f)))
+(define (is-substring? s sub)
+  (is-substring-expl? (explode s) (explode sub) (explode sub)))
+
+(define (is-substring-expl? s sub backup)
+  (cond [(empty? sub) #t]
+        [(empty? s) #f]
+        [(equal? (first s) (first sub))
+         (is-substring-expl? (rest s) (rest sub) backup)]
+        [(equal? (first s) (first backup))
+         (is-substring-expl? (rest s) (rest backup) backup)]
+        [else (is-substring-expl? (rest s) sub backup)]))
\ No newline at end of file