Miscellaneous improvements

manuscript/chapter4.txt

@@ -167,11 +167,11 @@ printf '' >file
 extract() {
     # Usage: extract file "opening marker" "closing marker"
     while IFS=$'\n' read -r line; do
-        [[ "$extract" && "$line" != "$3" ]] && \
+        [[ $extract && $line != "$3" ]] &&
             printf '%s\n' "$line"
 
-        [[ "$line" == "$2" ]] && extract=1
-        [[ "$line" == "$3" ]] && extract=
+        [[ $line == "$2" ]] && extract=1
+        [[ $line == "$3" ]] && extract=
     done < "$1"
 }
 ```