diff --git a/.github/run.sh b/.github/run.sh
index 60062fa..4079db4 100755
--- a/.github/run.sh
+++ b/.github/run.sh
@@ -35,7 +35,7 @@ if (( no_test == 0 )); then
   else
     green='\033[1;32m'
     no_color='\033[0m'
-	printf "Running tests in the package: %b%s%b\n" "$green" "$1" "$no_color"
+	printf "Running tests in packages matching: %b%s*%b\n" "$green" "$1" "$no_color"
     sbt -warn "Test / testOnly $1*"
   fi
 fi
diff --git a/README.md b/README.md
index 60f8cb5..6996fd5 100644
--- a/README.md
+++ b/README.md
@@ -100,15 +100,15 @@ P48 (**) Truth tables for logical expressions (3).
 
 P54 Omitted; our tree representation will only allow well-formed trees.
 
-P55 (**) Construct completely balanced binary trees.
+[P55](src/main/scala/bintree/P55.scala) (**) Construct completely balanced binary trees.
 
-P56 (**) Symmetric binary trees.
+[P56](src/main/scala/bintree/P56.scala) (**) Symmetric binary trees.
 
-P57 (**) Binary search trees (dictionaries).
+[P57](src/main/scala/bintree/P57.scala) (**) Binary search trees (dictionaries).
 
-P58 (**) Generate-and-test paradigm.
+[P58](src/main/scala/bintree/P58.scala) (**) Generate-and-test paradigm.
 
-P59 (**) Construct height-balanced binary trees.
+[P59](src/main/scala/bintree/P59.scala) (**) Construct height-balanced binary trees.
 
 P60 (**) Construct height-balanced binary trees with a given number of nodes.
 
diff --git a/src/main/scala/bintree/P55.scala b/src/main/scala/bintree/P55.scala
new file mode 100644
index 0000000..2d48b49
--- /dev/null
+++ b/src/main/scala/bintree/P55.scala
@@ -0,0 +1,45 @@
+package bintree
+
+import Tree.*
+
+// P55 (**) Construct completely balanced binary trees.
+//     In a completely balanced binary tree, the following property holds for
+//     every node: The number of nodes in its left subtree and the number of
+//     nodes in its right subtree are almost equal, which means their difference
+//     is not greater than one.
+//
+//     Define an object named Tree.  Write a function Tree.cBalanced to
+//     construct completely balanced binary trees for a given number of nodes.
+//     The function should generate all solutions.  The function should take as
+//     parameters the number of nodes and a single value to put in all of them.
+//
+//     scala> Tree.cBalanced(4, "x")
+//     res0: List(Node[String]) = List(T(x T(x . .) T(x . T(x . .))), T(x T(x . .) T(x T(x . .) .)), ...
+
+object P55:
+  private def build[A](n: Int, x: A): List[(Tree[A], Int)] =
+    if n == 0
+    then Nil
+    else if n == 1
+    then List((singleton(x), 1))
+    else if n == 2
+    then
+      List(
+        (Node(x, singleton(x), Empty), 2),
+        (Node(x, Empty, singleton(x)), 2)
+      )
+    else
+      val k = (n - 1) / 2
+      for
+        (l, i) <- build(k, x)
+        (r, j) <- build(n - k - 1, x)
+        h = i + j + 1
+        xs =
+          if math.abs(i - j) == 1
+          then List((Node(x, r, l), h))
+          else Nil
+        y <- (Node(x, l, r), h) :: xs
+      yield y
+
+  def cBalanced[A](n: Int, x: A): List[Tree[A]] =
+    build(n, x).map(_._1)
diff --git a/src/main/scala/bintree/P56.scala b/src/main/scala/bintree/P56.scala
new file mode 100644
index 0000000..2ede492
--- /dev/null
+++ b/src/main/scala/bintree/P56.scala
@@ -0,0 +1,29 @@
+package bintree
+
+import Tree.*
+
+// P56 (**) Symmetric binary trees.
+//     Let us call a binary tree symmetric if you can draw a vertical line
+//     through the root node and then the right subtree is the mirror image of
+//     the left subtree.  Add an isSymmetric method to the Tree class to check
+//     whether a given binary tree is symmetric.  Hint: Write an isMirrorOf
+//     method first to check whether one tree is the mirror image of another.
+//     We are only interested in the structure, not in the contents of the
+//     nodes.
+//
+//     scala> Node('a', Node('b'), Node('c')).isSymmetric
+//     res0: Boolean = true
+
+object P56:
+  private def isMirror[A](t1: Tree[A], t2: Tree[A]): Boolean =
+    (t1, t2) match
+      case (Empty, Empty) => true
+      case (Empty, _)     => false
+      case (_, Empty)     => false
+      case (Node(_, l1, r1), Node(_, l2, r2)) =>
+        isMirror(l1, r2) && isMirror(r1, l2)
+
+  extension [A](t: Tree[A])
+    def isSymmetric: Boolean = t match
+      case Empty         => true
+      case Node(_, l, r) => isMirror(l, r)
diff --git a/src/main/scala/bintree/P57.scala b/src/main/scala/bintree/P57.scala
new file mode 100644
index 0000000..c03dfc2
--- /dev/null
+++ b/src/main/scala/bintree/P57.scala
@@ -0,0 +1,52 @@
+package bintree
+
+import Tree.*
+import math.Ordered.orderingToOrdered
+
+// P57 (**) Binary search trees (dictionaries).
+//     Write a function to add an element to a binary search tree.
+//
+//     scala> End.addValue(2)
+//     res0: Node[Int] = T(2 . .)
+//
+//     scala> res0.addValue(3)
+//     res1: Node[Int] = T(2 . T(3 . .))
+//
+//     scala> res1.addValue(0)
+//     res2: Node[Int] = T(2 T(0 . .) T(3 . .))
+//
+//     Hint: The abstract definition of addValue in Tree should be
+//     `def addValue[U >: T <% Ordered[U]](x: U): Tree[U]`.  The `>: T` is
+//     because addValue's parameters need to be _contravariant_ in T.
+//     (Conceptually, we're adding nodes above existing nodes.  In order for the
+//     subnodes to be of type T or any subtype, the upper nodes must be of type
+//     T or any supertype.)  The `<% Ordered[U]` allows us to use the < operator
+//     on the values in the tree.
+//
+//     Use that function to construct a binary tree from a list of integers.
+//
+//     scala> Tree.fromList(List(3, 2, 5, 7, 1))
+//     res3: Node[Int] = T(3 T(2 T(1 . .) .) T(5 . T(7 . .)))
+//
+//     Finally, use that function to test your solution to P56.
+//
+//     scala> Tree.fromList(List(5, 3, 18, 1, 4, 12, 21)).isSymmetric
+//     res4: Boolean = true
+//
+//     scala> Tree.fromList(List(3, 2, 5, 7, 4)).isSymmetric
+//     res5: Boolean = false
+
+object P57:
+  extension [A](t: Tree[A])
+    // "covariant type A occurs in contravariant position".
+    // If we allow any subtype of A, then a list of cats
+    // may contain a dog!
+    // https://stackoverflow.com/a/43180701/839733
+    def addValue[B >: A](x: B)(using Ordering[B]): Tree[B] = t match
+      case Empty => singleton(x)
+      case Node(y, l, r) if x < y =>
+        val left = l.addValue(x)
+        Node(y, left, r)
+      case Node(y, l, r) =>
+        val right = r.addValue(x)
+        Node(y, l, right)
diff --git a/src/main/scala/bintree/P58.scala b/src/main/scala/bintree/P58.scala
new file mode 100644
index 0000000..976be8e
--- /dev/null
+++ b/src/main/scala/bintree/P58.scala
@@ -0,0 +1,14 @@
+package bintree
+
+import P56.*
+
+// P58 (**) Generate-and-test paradigm.
+//     Apply the generate-and-test paradigm to construct all symmetric,
+//     completely balanced binary trees with a given number of nodes.
+//
+//     scala> Tree.symmetricBalancedTrees(5, "x")
+//     res0: List[Node[String]] = List(T(x T(x . T(x . .)) T(x T(x . .) .)), T(x T(x T(x . .) .) T(x . T(x . .))))
+
+object P58:
+  def symmetricBalancedTrees[A](n: Int, x: A): List[Tree[A]] =
+    P55.cBalanced(n, x).filter(_.isSymmetric)
diff --git a/src/main/scala/bintree/P59.scala b/src/main/scala/bintree/P59.scala
new file mode 100644
index 0000000..42f6807
--- /dev/null
+++ b/src/main/scala/bintree/P59.scala
@@ -0,0 +1,39 @@
+package bintree
+
+import Tree.*
+
+// P59 (**) Construct height-balanced binary trees.
+//     In a height-balanced binary tree, the following property holds for every
+//     node: The height of its left subtree and the height of its right subtree
+//     are almost equal, which means their difference is not greater than one.
+//
+//     Write a method Tree.hbalTrees to construct height-balanced binary trees
+//     for a given height with a supplied value for the nodes.  The function
+//     should generate all solutions.
+//
+//     scala> Tree.hbalTrees(3, "x")
+//     res0: List[Node[String]] = List(T(x T(x T(x . .) T(x . .)) T(x T(x . .) T(x . .))), T(x T(x T(x . .) T(x . .)) T(x T(x . .) .)), ...
+
+object P59:
+  def hbalTrees[A](h: Int, x: A): List[Tree[A]] =
+    if h == 0
+    then Nil
+    else if h == 1
+    then List(singleton(x))
+    else if h == 2
+    then
+      List(
+        Node(x, singleton(x), Empty),
+        Node(x, Empty, singleton(x)),
+        Node(x, singleton(x), singleton(x))
+      )
+    else
+      for
+        a <- hbalTrees(h - 1, x)
+        b <- hbalTrees(h - 2, x)
+        c <- List(
+          Node(x, a, b),
+          Node(x, b, a),
+          Node(x, a, a)
+        )
+      yield c
diff --git a/src/main/scala/bintree/Tree.scala b/src/main/scala/bintree/Tree.scala
new file mode 100644
index 0000000..5fca7ac
--- /dev/null
+++ b/src/main/scala/bintree/Tree.scala
@@ -0,0 +1,11 @@
+package bintree
+
+enum Tree[+A]:
+  case Empty
+  case Node(value: A, left: Tree[A], right: Tree[A])
+
+object Tree:
+  def empty[A]: Tree[A] = Empty
+
+  def singleton[A](x: A): Tree[A] =
+    Node(x, Empty, Empty)
diff --git a/src/main/scala/list/P27.scala b/src/main/scala/list/P27.scala
index 7301e7b..8b1c3f0 100644
--- a/src/main/scala/list/P27.scala
+++ b/src/main/scala/list/P27.scala
@@ -35,5 +35,5 @@ object P27:
       case n :: next =>
         for
           xs  <- P26.combinations(n, l)
-          xxs <- group(next, l.filterNot(x => xs.exists(_ == x)))
+          xxs <- group(next, l.filterNot(xs.contains))
         yield xs :: xxs
diff --git a/src/test/scala/bintree/P55Suite.scala b/src/test/scala/bintree/P55Suite.scala
new file mode 100644
index 0000000..92bbabc
--- /dev/null
+++ b/src/test/scala/bintree/P55Suite.scala
@@ -0,0 +1,29 @@
+package bintree
+
+import munit.FunSuite
+import Tree.*
+
+class P55Suite extends FunSuite:
+  test("construct completely balanced binary trees"):
+    val data = List(
+      (0, Nil),
+      (1, List(singleton('x'))),
+      (2, List(Node('x', singleton('x'), Empty), Node('x', Empty, singleton('x')))),
+      (3, (List(Node('x', singleton('x'), singleton('x'))))),
+      (
+        4,
+        List(
+          Node('x', singleton('x'), Node('x', singleton('x'), Empty)),
+          Node('x', Node('x', singleton('x'), Empty), singleton('x')),
+          Node('x', singleton('x'), Node('x', Empty, singleton('x'))),
+          Node('x', Node('x', Empty, singleton('x')), singleton('x'))
+        )
+      )
+    )
+    data.foreach { (n, expected) =>
+      val obtained = P55.cBalanced(n, 'x')
+      assertEquals(obtained.size, expected.size)
+      obtained.foreach { tree =>
+        assert(expected.contains(tree), s"Tree: $tree")
+      }
+    }
diff --git a/src/test/scala/bintree/P56Suite.scala b/src/test/scala/bintree/P56Suite.scala
new file mode 100644
index 0000000..a3715ad
--- /dev/null
+++ b/src/test/scala/bintree/P56Suite.scala
@@ -0,0 +1,13 @@
+package bintree
+
+import munit.FunSuite
+import Tree.*
+import P56.*
+
+class P56Suite extends FunSuite:
+  test("check whether a given binary tree is symmetric"):
+    val obtained = Node('a', singleton('b'), Empty).isSymmetric
+    assertEquals(obtained, false)
+
+    val obtained2 = Node('a', singleton('b'), singleton('c')).isSymmetric
+    assertEquals(obtained2, true)
diff --git a/src/test/scala/bintree/P57Suite.scala b/src/test/scala/bintree/P57Suite.scala
new file mode 100644
index 0000000..b717403
--- /dev/null
+++ b/src/test/scala/bintree/P57Suite.scala
@@ -0,0 +1,16 @@
+package bintree
+
+import munit.FunSuite
+import Tree.*
+import P56.*
+import P57.*
+
+class P57Suite extends FunSuite:
+  test("add an element to a BST"):
+    val xs       = List(3, 2, 5, 7, 1)
+    val obtained = xs.foldLeft(Tree.empty[Int])((t, x) => t.addValue(x))
+    assertEquals(obtained.isSymmetric, true)
+
+    val ys        = List(3, 2, 5, 7, 4)
+    val obtained2 = ys.foldLeft(Tree.empty[Int])((t, x) => t.addValue(x))
+    assertEquals(obtained2.isSymmetric, false)
diff --git a/src/test/scala/bintree/P58Suite.scala b/src/test/scala/bintree/P58Suite.scala
new file mode 100644
index 0000000..9cbb8c2
--- /dev/null
+++ b/src/test/scala/bintree/P58Suite.scala
@@ -0,0 +1,16 @@
+package bintree
+
+import munit.FunSuite
+import Tree.*
+
+class P58Suite extends FunSuite:
+  test("construct all symmetric, completely balanced binary trees with a given number of nodes"):
+    val obtained = P58.symmetricBalancedTrees(5, 'x')
+    val expected = List(
+      Node('x', Node('x', Empty, singleton('x')), Node('x', singleton('x'), Empty)),
+      Node('x', Node('x', singleton('x'), Empty), Node('x', Empty, singleton('x')))
+    )
+    assertEquals(obtained.size, expected.size)
+    obtained.foreach { tree =>
+      assert(expected.contains(tree), s"Tree: $tree")
+    }
diff --git a/src/test/scala/bintree/P59Suite.scala b/src/test/scala/bintree/P59Suite.scala
new file mode 100644
index 0000000..6171e6d
--- /dev/null
+++ b/src/test/scala/bintree/P59Suite.scala
@@ -0,0 +1,17 @@
+package bintree
+
+import munit.FunSuite
+import Tree.*
+
+class P59Suite extends FunSuite:
+  test("construct height-balanced binary trees"):
+    val obtained = P59.hbalTrees(3, 'x')
+    val expected = List(
+      Node('x', Node('x', Empty, Empty), Node('x', Empty, singleton('x'))),
+      Node('x', Node('x', Empty, Empty), Node('x', singleton('x'), Empty)),
+      Node('x', Node('x', Empty, Empty), Node('x', singleton('x'), singleton('x'))),
+      Node('x', Node('x', Empty, singleton('x')), Node('x', Empty, Empty))
+    )
+    expected.foreach { tree =>
+      assert(obtained.contains(tree), s"Tree: $tree")
+    }
diff --git a/src/test/scala/list/P23Suite.scala b/src/test/scala/list/P23Suite.scala
index 8ec5ef5..faaa6cd 100644
--- a/src/test/scala/list/P23Suite.scala
+++ b/src/test/scala/list/P23Suite.scala
@@ -12,12 +12,12 @@ class P23Suite extends ScalaCheckSuite:
       val k        = lst.size
       val n        = Random.nextInt(k + 1)
       val obtained = P23.randomSelect(n, lst)
-      assert(obtained.size == n && obtained.forall(x => lst.exists(_ == x)))
+      assert(obtained.size == n && obtained.forall(lst.contains))
     }
 
   property("extract n random elements from a list where n >>> list.size"):
     forAllNoShrink(Gen.listOf(ListGen.genInt)) { (lst: List[Int]) =>
       val k        = lst.size
       val obtained = P23.randomSelect(Int.MaxValue, lst)
-      assert(obtained.size == k && obtained.forall(x => lst.exists(_ == x)))
+      assert(obtained.size == k && obtained.forall(lst.contains))
     }