add difficulty description

Src/Problem1.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 1
-Find the last element of a list.
+(Easy) Find the last element of a list.
 -/
 variable {α : Type}
 

Src/Problem10.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 10
-Use the result of Problem 9 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists `(N, E)` where `N` is the number of duplicates of the element `E`.
+(Easy) Use the result of Problem 9 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists `(N, E)` where `N` is the number of duplicates of the element `E`.
 -/
 namespace P10 --#
 

Src/Problem11.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 11
-Modify the result of problem 10 in such a way that if an element has no duplicates it is simply copied into the result list. Only elements with duplicates are transferred as `(N E)` lists.
+(Easy) Modify the result of problem 10 in such a way that if an element has no duplicates it is simply copied into the result list. Only elements with duplicates are transferred as `(N E)` lists.
 -/
 namespace P11 --#
 

Src/Problem12.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 12
-Given a run-length code list generated as specified in problem 11. Construct its uncompressed version.
+(Intermediate) Given a run-length code list generated as specified in problem 11. Construct its uncompressed version.
 -/
 namespace P12 --#
 

Src/Problem13.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 13
-Implement the so-called run-length encoding data compression method directly. I.e. don't explicitly create the sublists containing the duplicates, as in problem 9, but only count them. As in problem P11, simplify the result list by replacing the singleton lists `(1 X)` by `X`.
+(Intermediate) Implement the so-called run-length encoding data compression method directly. I.e. don't explicitly create the sublists containing the duplicates, as in problem 9, but only count them. As in problem P11, simplify the result list by replacing the singleton lists `(1 X)` by `X`.
 -/
 namespace P13 --#
 

Src/Problem14.lean

@@ -1,6 +1,6 @@
 /-
-  # Problem 14
-  Duplicate the elements of a list.
+# Problem 14
+(Easy) Duplicate the elements of a list.
 -/
 
 variable {α : Type}

Src/Problem15.lean

@@ -1,6 +1,6 @@
 /-
-  # Problem 15
-  Replicate the elements of a list a given number of times.
+# Problem 15
+(Intermediate) Replicate the elements of a list a given number of times.
 -/
 
 variable {α : Type}

Src/Problem16.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 16
-Drop every N'th element from a list.
+(Intermediate) Drop every N'th element from a list.
 -/
 variable {α : Type}
 

Src/Problem17.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 17
-Split a list into two parts; the length of the first part is given.
+(Easy) Split a list into two parts; the length of the first part is given.
 -/
 variable {α : Type}
 

Src/Problem2.lean

@@ -1,7 +1,6 @@
 /-
 # Problem 2
-
-Find the last-but-one (or second-last) element of a list.
+(Easy) Find the last-but-one (or second-last) element of a list.
 -/
 variable {α : Type}
 

Src/Problem20.lean

@@ -1,6 +1,6 @@
 /-
-  # Problem 20
-  Remove the K'th element from a list.
+# Problem 20
+(Easy) Remove the K'th element from a list.
 -/
 variable {α : Type}
 

Src/Problem3.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 3
-Find the K'th element of a list.
+(Easy) Find the K'th element of a list.
 -/
 variable {α : Type}
 

Src/Problem31.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 31
-Determine whether a given integer number is prime.
+(Intermediate) Determine whether a given integer number is prime.
 -/
 
 def isPrime (n : Nat) : Bool :=

Src/Problem32.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 32
-Determine the greatest common divisor of two positive integer numbers. Use the Euclid's algorithm.
+(Intermediate) Determine the greatest common divisor of two positive integer numbers. Use the Euclid's algorithm.
 -/
 
 /-- Euclidean algorithm -/

Src/Problem4.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 4
-Find the number of elements in a list.
+(Easy) Find the number of elements in a list.
 -/
 variable {α : Type}
 

Src/Problem5.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 5
-Reverse a list.
+(Easy) Reverse a list.
 -/
 variable {α : Type}
 

Src/Problem6.lean

@@ -1,7 +1,6 @@
 /-
 # Problem 6
-
-Find out whether a list is a palindrome.
+(Easy) Find out whether a list is a palindrome.
 
 Hint: A palindrome can be read forward or backward; e.g. (x a m a x).
 -/

Src/Problem7.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 7
-Flatten a nested list structure.
+(Intermediate) Flatten a nested list structure.
 -/
 
 -- We have to define a new data type, because lists in Lean are homogeneous.

Src/Problem8.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 8
-Eliminate consecutive duplicates of list elements.
+(Intermediate) Eliminate consecutive duplicates of list elements.
 -/
 variable {α : Type} [BEq α]
 

Src/Problem9.lean

@@ -1,6 +1,6 @@
 /-
 # Problem 9
-Pack consecutive duplicates of list elements into sublists.
+(Intermediate) Pack consecutive duplicates of list elements into sublists.
 -/
 namespace P9 --#