diff --git a/README.md b/README.md
index e1fccbc..7a33b1e 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,9 @@ Solutions to https://adventofcode.com/2023/
 
 This is my first experience with Lisp, so the code is probably terrible.
 
+Also I decided to reimplement all the mostly basic stuff (like `map` or `reduce`) myself,
+both for fun and because Guile documentation is really bad.
+
 Requirements: Guile 3 in `/usr/bin`.
 
 In a folder for a specific day,
diff --git a/day01-hard/main.scm b/day01-hard/main.scm
index fa0ec5f..87ee9a0 100755
--- a/day01-hard/main.scm
+++ b/day01-hard/main.scm
@@ -1,6 +1,7 @@
 #!/usr/bin/guile -s
 !#
 
+;=============== functions with side effects (I/O) ======================
 (use-modules (ice-9 rdelim))
 
 (define (read-lines)
@@ -8,17 +9,18 @@
     (if (eof-object? line) '() (cons line (read-lines))))
   (read-line)))
 
+(define (display-ln value) (list (display value) (display "\n")))
+
 (define (assert-eq error-message expected actual)
   (if (not (equal? expected actual))
-    (list
-      (display (list error-message (list "expected" expected) (list "actual" actual)))
-      (display "\n"))
+    (display-ln (list error-message (list "expected" expected) (list "actual" actual)))
     '()))
 
-(define (tee value) (car (list value (display value) (display "\n"))))
+(define (tee value) (car (list value (display-ln value))))
+;=============== end of functions with side effects ======================
 
 (define (id value) value)
-(define (is-not value) (lambda (x) (not (= x value))))
+(define (is-not value) (lambda (x) (not (equal? x value))))
 (define is-not-empty (lambda (x) (not (null? x))))
 
 (define (prepend first-value) (lambda (rest) (cons first-value rest)))