diff --git a/ar.bash b/ar.bash
index c887aa6..e49ff5c 100644
--- a/ar.bash
+++ b/ar.bash
@@ -167,7 +167,7 @@ ar::insert() {
 
 
 ## @fn ar::index()
-## @brief Return the index of the first item in array whose needle is equal to the given value.
+## @brief Return the index of the first item in array equal to the given value.
 ## @detail Like Python's list.index(x[, start[, end]])
 ## Usage: ar::index arrayname needle [start [end]]
 ar::index() {
@@ -304,8 +304,8 @@ ar::reverse() {
 ## @param arr_set The array to write the result into
 ar::set() {
     local -n arr="$1"
-    local -A assoc
     local -n arr_set="$2"
+    local -A assoc
     for val in "${arr[@]}"; do
 	assoc[$val]=1
     done
@@ -351,12 +351,12 @@ ar::union() {
 }
 
 
-## @fn ar::punion()
+## @fn ar::print_union()
 ## @brief Print the union of two arrays
 ## @detail Can be captured as an array with res=($(punion arr1 arr2))
 ## @param arr1 The first array
 ## @param arr2 The second array
-ar::punion() {
+ar::print_union() {
   local -n arr1="$1"
   local -n arr2="$2"
   local -A union_assoc
@@ -399,6 +399,62 @@ ar::intersection() {
     intersect_arr=("${!intersect_assoc[@]}")
 }
 
+## @fn ar::difference
+## @brief Write the difference between two sets to a third var
+## @detail Write elements that are in the first set but not the second.
+## @param arr1 The first array name
+## @param arr2 The second array name
+## @params difference_arr The array name to write the result to
+ar::difference() {
+    local -n arr1="$1"
+    local -n arr2="$2"
+    local -n difference_arr="$3"
+    local -A assoc2
+    local -A diff_assoc
+    for val in "${arr2[@]}"; do
+	assoc2["$val"]=1
+    done
+    for val in "${arr1[@]}"; do
+	if [[ ! -v assoc2["$val"] ]]; then
+	    diff_assoc["$val"]=1
+	fi
+    done
+    difference_arr=("${!diff_assoc[@]}")
+}
+
+## @fn ar::symmetric_difference
+## @brief Write the symmetric difference between two sets to a third var
+## @detail Write elements are in either set but not their intersection
+## @param arr1 The first array name
+## @param arr2 The second array name
+## @params difference_arr The array name to write the result to
+ar::symmetric_difference() {
+    local -n arr1="$1"
+    local -n arr2="$2"
+    local -n symmetric_diff_arr="$3"
+    local -A assoc1 assoc2 symmetric_diff_assoc
+
+    for val in "${arr1[@]}"; do
+	assoc1["$val"]=1
+    done
+    for val in "${arr2[@]}"; do
+	assoc2["$val"]=1
+    done
+
+    # Find elements in arr1 but not arr2
+    for val in "${arr1[@]}"; do
+	if [[ ! -v assoc2["$val"] ]]; then
+	    symmetric_diff_assoc["$val"]=1
+	fi
+    done
+    # Find elements in arr2 but not arr1
+    for val in "${arr2[@]}"; do
+	if [[ ! -v assoc1["$val"] ]]; then
+	    symmetric_diff_assoc["$val"]=1
+	fi
+    done
+    symmetric_diff_arr=("${!symmetric_diff_assoc[@]}")
+}
 
 ## @fn array_to_string
 ## @brief Turn an array into a string with seperator
diff --git a/tests/test_ar.bats b/tests/test_ar.bats
index 89649d7..69f5073 100644
--- a/tests/test_ar.bats
+++ b/tests/test_ar.bats
@@ -38,6 +38,11 @@ setup() {
     done
 }
 
+@test "ar::pop removes and returns an element from the end of the array" {
+    ar::pop _starting_array
+    [[ "${#_starting_array[@]}" == 2 ]]
+}
+
 @test "ar::index returns the index for the first occurence of value" {
     local i
     i="$(ar::index _starting_array sausage)"
@@ -54,6 +59,12 @@ setup() {
     [[ $status -eq 2 ]]
 }
 
+@test "ar::extend appends items from the second array to the first array" {
+    local _second_array=(onions celery garlic)
+    ar::extend _starting_array _second_array
+    [[ "${_starting_array[*]}" == "rice beans sausage onions celery garlic" ]]
+}
+
 @test "ar::remove removes the first occurence of value from the array" {
     ar::remove _starting_array rice
     [[ "${_starting_array[*]}" == "beans sausage" ]]
@@ -61,7 +72,25 @@ setup() {
 
 @test "ar::remove fails with non-zero rc if value does not exist" {
     run ar::remove _starting_array beef
-    [[ $status -eq 1 ]]
+    assert_failure
+}
+
+@test "ar::count returns the number of times value appears in array" {
+    run ar::count _starting_array "rice"
+    assert_success
+    assert_output "1"
+
+    local _another_array=(rice beans rice sausage)
+    run ar::count _another_array "rice"
+    assert_success
+    assert_output "2"
+}
+
+@test "ar::reverse reverses an array in-place" {
+    local _starting_array=(rice beans sausage)
+    local _expected_reversed_array=(sausage beans rice)
+    ar::reverse _starting_array
+    assert_equal "${_starting_array[*]}" "${_expected_reversed_array[*]}"
 }
 
 @test "ar::set turns an array into a set array" {
@@ -77,3 +106,55 @@ setup() {
 	ar::in_set expected_arr "${set_arr[i]}"
     done
 }
+
+@test "ar::union writes the union of two sets to a third var" {
+    local -a first_arr=(rice beans sausage)
+    local -a second_arr=(beef onions)
+    local -i first_len="${#first_arr[@]}"
+    local -i second_len="${#second_arr[@]}"
+    local -i combined_len=$(( first_len + second_len ))
+    local -i i
+    local -a union_arr
+    ar::union first_arr second_arr union_arr
+    assert_equal "$combined_len" 5
+    for ((i=0; i < first_len; i++)); do
+	ar::in_set union_arr "${first_arr[i]}"
+	[[ $status -eq 0 ]]
+    done
+    for ((i=0; i < second_len; i++)); do
+	ar::in_set union_arr "${second_arr[i]}"
+	[[ $status -eq 0 ]]
+    done
+}
+
+@test "ar::intersection writes the intersection of two sets to a third var" {
+    local -a first_arr=(rice beans sausage)
+    local -a second_arr=(rice beef onions)
+    local -a intersection_arr
+    ar::intersection first_arr second_arr intersection_arr
+    assert_equal "${intersection_arr[0]}" "rice"
+}
+
+@test "ar::difference writes the set difference of two sets to a third var" {
+    local -a first_arr=(rice beans sausage)
+    local -a second_arr=(rice beef onions)
+    local -a expected_diff=(beans sausage)
+    local -a diff_arr
+    ar::difference first_arr second_arr diff_arr
+    assert_equal "${diff_arr[*]}" "${expected_diff[*]}"
+}
+
+@test "ar::symmetric_difference writes the symmetric set difference of two sets to a third var" {
+    local -a first_arr=(rice beans sausage)
+    local -a second_arr=(rice beef onions)
+    local -a expected_diff=(beans sausage beef onions)
+    local -a diff_arr
+    ar::symmetric_difference first_arr second_arr diff_arr
+    # Order shouldn't matter
+    local -i expected_len="${#expected_diff[@]}"
+    local -i actual_len="${#diff_arr[@]}"
+    assert_equal "$expected_len" "$actual_len"
+    for ((i=0; i < expected_len; i++)); do
+	ar::in_set diff_arr "${expected_diff[i]}"
+    done
+}