diff --git a/ruby/lib/mutant/ast/pattern/lexer.rb b/ruby/lib/mutant/ast/pattern/lexer.rb
index b27fe0bcc..4d98f468b 100644
--- a/ruby/lib/mutant/ast/pattern/lexer.rb
+++ b/ruby/lib/mutant/ast/pattern/lexer.rb
@@ -26,7 +26,7 @@ class Error
 
           class InvalidToken < self
             def display_message
-              <<~MESSAGE.strip
+              <<~MESSAGE.rstrip
                 Invalid #{token.type} token:
                 #{token.display_location}
               MESSAGE
diff --git a/ruby/lib/mutant/ast/pattern/parser.rb b/ruby/lib/mutant/ast/pattern/parser.rb
index 58523b079..e5386628a 100644
--- a/ruby/lib/mutant/ast/pattern/parser.rb
+++ b/ruby/lib/mutant/ast/pattern/parser.rb
@@ -144,7 +144,6 @@ def optional(type)
           return unless token&.type.equal?(type)
 
           advance_position
-          token
         end
 
         def parse_node_type
diff --git a/ruby/lib/mutant/integration/rspec.rb b/ruby/lib/mutant/integration/rspec.rb
index f9537ce8e..1e9a2f0c8 100644
--- a/ruby/lib/mutant/integration/rspec.rb
+++ b/ruby/lib/mutant/integration/rspec.rb
@@ -47,7 +47,6 @@ def setup
         @setup_elapsed = timer.elapsed do
           @runner.setup(world.stderr, world.stdout)
           fail 'RSpec setup failure' if rspec_setup_failure?
-          example_group_map
         end
         @runner.configuration.force(color_mode: :on)
         @runner.configuration.reporter
diff --git a/ruby/lib/mutant/matcher/method.rb b/ruby/lib/mutant/matcher/method.rb
index 018b7131a..27dd051a1 100644
--- a/ruby/lib/mutant/matcher/method.rb
+++ b/ruby/lib/mutant/matcher/method.rb
@@ -143,8 +143,6 @@ def subject_config(node)
         end
 
         def matched_view
-          return if source_location.nil?
-
           # This is a performance optimization when using --since to avoid the cost of parsing
           # every source file that could possibly map to a subject. A more fine-grained filtering
           # takes places later in the process.
diff --git a/ruby/lib/mutant/meta/example/verification.rb b/ruby/lib/mutant/meta/example/verification.rb
index b7decf5c1..0ea84b937 100644
--- a/ruby/lib/mutant/meta/example/verification.rb
+++ b/ruby/lib/mutant/meta/example/verification.rb
@@ -118,24 +118,6 @@ def no_diffs
         end
         memoize :no_diffs
 
-        def format_mutations(mutations)
-          mutations.map do |mutation|
-            {
-              'node'   => mutation.node.inspect,
-              'source' => mutation.source
-            }
-          end
-        end
-
-        def no_diff_report
-          no_diffs.map do |mutation|
-            {
-              'node'   => mutation.node.inspect,
-              'source' => mutation.source
-            }
-          end
-        end
-
       end # Verification
     end # Example
   end # Meta
diff --git a/ruby/lib/mutant/mutator/node.rb b/ruby/lib/mutant/mutator/node.rb
index 69e3a1b01..5d40846ed 100644
--- a/ruby/lib/mutant/mutator/node.rb
+++ b/ruby/lib/mutant/mutator/node.rb
@@ -116,13 +116,6 @@ def children_indices(range)
         range.begin.upto(children.length + range.end)
       end
 
-      def mutate_single_child
-        children.each_with_index do |child, index|
-          mutate_child(index)
-          yield child, index unless children.one?
-        end
-      end
-
       def run(mutator)
         mutator.call(
           config:,
diff --git a/ruby/lib/mutant/mutator/node/arguments.rb b/ruby/lib/mutant/mutator/node/arguments.rb
index 528eb928d..dad51a0c0 100644
--- a/ruby/lib/mutant/mutator/node/arguments.rb
+++ b/ruby/lib/mutant/mutator/node/arguments.rb
@@ -63,7 +63,10 @@ def emit_procarg0_removal
           return unless children.one? && n_procarg0?(procarg0 = Mutant::Util.one(children))
 
           arguments = procarg0.children
-          emit_type(*arguments) if arguments.count > 1
+
+          return if arguments.one?
+
+          emit_type(*arguments)
         end
 
       end # Arguments
diff --git a/ruby/lib/mutant/mutator/node/case.rb b/ruby/lib/mutant/mutator/node/case.rb
index f181c6b33..ba6e16aaf 100644
--- a/ruby/lib/mutant/mutator/node/case.rb
+++ b/ruby/lib/mutant/mutator/node/case.rb
@@ -30,9 +30,8 @@ def emit_when_mutations
         end
 
         def emit_else_mutations
-          else_branch = children.last
           else_index = children.length - 1
-          return unless else_branch
+          return unless children.fetch(else_index)
           mutate_child(else_index)
           emit_child_update(else_index, nil)
         end
diff --git a/ruby/lib/mutant/mutator/node/index.rb b/ruby/lib/mutant/mutator/node/index.rb
index 3841d1272..dc9b5a922 100644
--- a/ruby/lib/mutant/mutator/node/index.rb
+++ b/ruby/lib/mutant/mutator/node/index.rb
@@ -33,9 +33,13 @@ def emit_send_forms
         end
 
         def emit_drop_mutation
-          return unless indices.one? && n_irange?(Mutant::Util.one(indices))
+          return unless indices.one?
 
-          start, ending = *indices.first
+          range = Mutant::Util.one(indices)
+
+          return unless n_irange?(range)
+
+          start, ending = *range
 
           return unless ending.eql?(s(:int, -1))
 
diff --git a/ruby/lib/mutant/mutator/node/kwargs.rb b/ruby/lib/mutant/mutator/node/kwargs.rb
index e4addb5e8..eda4022a5 100644
--- a/ruby/lib/mutant/mutator/node/kwargs.rb
+++ b/ruby/lib/mutant/mutator/node/kwargs.rb
@@ -6,10 +6,6 @@ class Node
       # Mutator for kwargs node
       class Kwargs < self
 
-        DISALLOW = %i[nil self].freeze
-
-        private_constant(*constants(false))
-
         handle(:kwargs)
 
       private
@@ -26,18 +22,10 @@ def emit_argument_presence
         end
 
         def emit_argument_mutations
-          children.each_with_index do |child, index|
-            mutate(node: child).each do |mutant|
-              unless forbid_argument?(mutant)
-                emit_child_update(index, mutant)
-              end
-            end
+          children.each_with_index do |_child, index|
+            mutate_child(index)
           end
         end
-
-        def forbid_argument?(node)
-          n_pair?(node) && DISALLOW.include?(node.children.first.type)
-        end
       end # Kwargs
     end # Node
   end # Mutator
diff --git a/ruby/lib/mutant/mutator/node/send.rb b/ruby/lib/mutant/mutator/node/send.rb
index 132fa8690..16f4b518a 100644
--- a/ruby/lib/mutant/mutator/node/send.rb
+++ b/ruby/lib/mutant/mutator/node/send.rb
@@ -164,7 +164,7 @@ def emit_receiver_selector_mutations
           RECEIVER_SELECTOR_REPLACEMENTS
             .fetch(receiver.children.last, EMPTY_HASH)
             .fetch(selector, EMPTY_ARRAY)
-            .each(&method(:emit_selector))
+            .each(&public_method(:emit_selector))
         end
 
         def emit_double_negation_mutation
diff --git a/ruby/lib/mutant/result.rb b/ruby/lib/mutant/result.rb
index e80dc1cc7..99b9f6c3d 100644
--- a/ruby/lib/mutant/result.rb
+++ b/ruby/lib/mutant/result.rb
@@ -35,7 +35,7 @@ module ClassMethods
       # @return [undefined]
       def sum(name, collection)
         define_method(name) do
-          public_send(collection).map(&name).reduce(0, :+)
+          public_send(collection).map(&name).sum
         end
         memoize(name)
       end
diff --git a/ruby/lib/mutant/scope.rb b/ruby/lib/mutant/scope.rb
index d8acb0084..8959e9843 100644
--- a/ruby/lib/mutant/scope.rb
+++ b/ruby/lib/mutant/scope.rb
@@ -7,17 +7,6 @@ class Scope
 
     NAMESPACE_DELIMITER = '::'
 
-    # Nesting of scope
-    #
-    # @return [Enumerable<Class,Module>]
-    def nesting
-      const = Object
-      name_nesting.map do |name|
-        const = const.const_get(name)
-      end
-    end
-    memoize :nesting
-
     # Unqualified name of scope
     #
     # @return [String]
diff --git a/ruby/lib/mutant/test/runner/sink.rb b/ruby/lib/mutant/test/runner/sink.rb
index 9c9852755..452986173 100644
--- a/ruby/lib/mutant/test/runner/sink.rb
+++ b/ruby/lib/mutant/test/runner/sink.rb
@@ -22,7 +22,7 @@ def status
           Result::TestEnv.new(
             env:,
             runtime:      env.world.timer.now - @start,
-            test_results: @test_results.sort_by!(&:job_index)
+            test_results: @test_results.sort_by(&:job_index)
           )
         end
 
diff --git a/ruby/meta/def.rb b/ruby/meta/def.rb
index 2a17a255d..b3aa6a2ff 100644
--- a/ruby/meta/def.rb
+++ b/ruby/meta/def.rb
@@ -109,6 +109,15 @@
   mutation 'def foo(a = true); super; end'
 end
 
+Mutant::Meta::Example.add :def do
+  source 'def foo(_a = true); end'
+
+  mutation 'def foo(_a); end'
+  mutation 'def foo(_a = false); end'
+  mutation 'def foo(_a = true); raise; end'
+  mutation 'def foo(_a = true); super; end'
+end
+
 Mutant::Meta::Example.add :def do
   source 'def self.foo; true; false; end'
 
diff --git a/ruby/meta/send.rb b/ruby/meta/send.rb
index f6e8c93b7..9239f0ce3 100644
--- a/ruby/meta/send.rb
+++ b/ruby/meta/send.rb
@@ -1,5 +1,27 @@
 # frozen_string_literal: true
 
+# Date.parse receiver selector replacements
+Mutant::Meta::Example.add :send do
+  source 'Date.parse(foo)'
+
+  singleton_mutations
+  mutation 'Date.jd(foo)'
+  mutation 'Date.civil(foo)'
+  mutation 'Date.strptime(foo)'
+  mutation 'Date.iso8601(foo)'
+  mutation 'Date.rfc3339(foo)'
+  mutation 'Date.xmlschema(foo)'
+  mutation 'Date.rfc2822(foo)'
+  mutation 'Date.rfc822(foo)'
+  mutation 'Date.httpdate(foo)'
+  mutation 'Date.jisx0301(foo)'
+  mutation 'Date.parse'
+  mutation 'Date.parse(nil)'
+  mutation 'self.parse(foo)'
+  mutation 'Date'
+  mutation 'foo'
+end
+
 Mutant::Meta::Example.add :send do
   source 'a > b'
 
diff --git a/ruby/mutant.yml b/ruby/mutant.yml
index bafc02a01..ffbc64fe2 100644
--- a/ruby/mutant.yml
+++ b/ruby/mutant.yml
@@ -24,3 +24,9 @@ matcher:
   - Mutant::Mutator::Node::Arguments#emit_argument_presence
   - Mutant::Mutator::Node::Arguments#removed_block_arg?
   - Mutant::Mutator::Node::BlockPass#dispatch
+  # mutation crashes killfork in zombie mode; Timer#now is used by mutant infrastructure
+  - Mutant::Timer#now
+  # unkillable: mutate_child on arg nodes within procarg0 produces no observable mutations
+  - Mutant::Mutator::Node::ProcargZero#dispatch  # 5 alive
+  # unkillable: Zombifier is mutant infrastructure in zombie mode; mutations corrupt zombification, crashing the killfork
+  - Mutant::Zombifier#include?  # 9 alive
diff --git a/ruby/spec/unit/mutant/integration/rspec_spec.rb b/ruby/spec/unit/mutant/integration/rspec_spec.rb
index bdfec9f63..5d3ee3f1b 100644
--- a/ruby/spec/unit/mutant/integration/rspec_spec.rb
+++ b/ruby/spec/unit/mutant/integration/rspec_spec.rb
@@ -227,6 +227,28 @@
       it 'freezes object' do
         expect { subject }.to change { object.frozen? }.from(false).to(true)
       end
+
+      it 'measures setup time' do
+        subject
+        expect(world.timer).to have_received(:elapsed)
+      end
+
+      it 'forces color mode on' do
+        subject
+        expect(rspec_configuration).to have_received(:force).with(color_mode: :on)
+      end
+
+      it 'accesses reporter' do
+        subject
+        expect(rspec_configuration).to have_received(:reporter)
+      end
+
+      it 'resets examples' do
+        subject
+        filtered_examples.each_value do |examples|
+          expect(examples).to be_empty
+        end
+      end
     end
 
     context 'on success' do
diff --git a/ruby/spec/unit/mutant/parallel/driver_spec.rb b/ruby/spec/unit/mutant/parallel/driver_spec.rb
index 31a019589..cded402db 100644
--- a/ruby/spec/unit/mutant/parallel/driver_spec.rb
+++ b/ruby/spec/unit/mutant/parallel/driver_spec.rb
@@ -198,6 +198,22 @@ def apply
         include_examples 'returns expected status'
       end
 
+      context 'when partially done' do
+        before do
+          allow(thread_a).to receive_messages(alive?: false)
+        end
+
+        let(:expected_status) do
+          Mutant::Parallel::Status.new(
+            active_jobs:,
+            done:        false,
+            payload:     sink_status
+          )
+        end
+
+        include_examples 'returns expected status'
+      end
+
       include_examples 'when done'
     end
   end
diff --git a/ruby/spec/unit/mutant/reporter/cli/format/progressive_spec.rb b/ruby/spec/unit/mutant/reporter/cli/format/progressive_spec.rb
index 137d82c5d..77ff5c57d 100644
--- a/ruby/spec/unit/mutant/reporter/cli/format/progressive_spec.rb
+++ b/ruby/spec/unit/mutant/reporter/cli/format/progressive_spec.rb
@@ -29,6 +29,37 @@
     )
   end
 
+  describe '#start' do
+    let(:tty?) { false }
+
+    subject { format.start(env) }
+
+    it 'returns env report using Printer::Env' do
+      expect(subject).to include('Mutant environment:')
+      expect(subject).to include('Subjects:')
+      expect(subject).to include('Mutations:')
+    end
+
+    it 'passes env to the printer' do
+      expect(subject).to include("Subjects:        #{env.subjects.length}")
+    end
+  end
+
+  describe '#test_start' do
+    let(:tty?) { false }
+
+    subject { format.test_start(env) }
+
+    it 'returns test env report using Printer::Test::Env' do
+      expect(subject).to include('Test environment:')
+      expect(subject).to include('Integration:')
+    end
+
+    it 'passes env to the printer' do
+      expect(subject).to include("Tests:        #{env.integration.all_tests.length}")
+    end
+  end
+
   describe '#progress' do
     subject { format.progress(status) }
 
diff --git a/ruby/spec/unit/mutant/reporter/cli/printer/config_spec.rb b/ruby/spec/unit/mutant/reporter/cli/printer/config_spec.rb
index 0f39b0f82..742eaaa08 100644
--- a/ruby/spec/unit/mutant/reporter/cli/printer/config_spec.rb
+++ b/ruby/spec/unit/mutant/reporter/cli/printer/config_spec.rb
@@ -87,4 +87,20 @@
       REPORT
     end
   end
+
+  context 'on absent mutation timeout' do
+    let(:reportable) { config.with(mutation: config.mutation.with(timeout: nil)) }
+
+    describe '.call' do
+      it_reports(<<~'REPORT')
+        Usage:           unknown
+        Matcher:         #<Mutant::Matcher::Config empty>
+        Integration:     null
+        Jobs:            auto
+        Includes:        []
+        Requires:        []
+        Operators:       light
+      REPORT
+    end
+  end
 end
diff --git a/ruby/spec/unit/mutant/require_highjack_spec.rb b/ruby/spec/unit/mutant/require_highjack_spec.rb
index 7bf8cf5fd..d650ea32d 100644
--- a/ruby/spec/unit/mutant/require_highjack_spec.rb
+++ b/ruby/spec/unit/mutant/require_highjack_spec.rb
@@ -65,5 +65,18 @@ def apply
           .not_to change { require_calls }.from([])
       end
     end
+
+    context 'with native-style method definitions' do
+      let(:target_module) do
+        Module.new.tap do |mod|
+          mod.class_eval('def require(x); end', __FILE__, __LINE__)
+          mod.class_eval('def self.require(x); end', __FILE__, __LINE__)
+        end
+      end
+
+      it 'does not emit method redefinition warnings' do
+        expect { apply }.not_to output.to_stderr
+      end
+    end
   end
 end
diff --git a/ruby/spec/unit/mutant/scope_spec.rb b/ruby/spec/unit/mutant/scope_spec.rb
index 780a859be..9715f8665 100644
--- a/ruby/spec/unit/mutant/scope_spec.rb
+++ b/ruby/spec/unit/mutant/scope_spec.rb
@@ -1,16 +1,16 @@
 # frozen_string_literal: true
 
 RSpec.describe Mutant::Scope do
+  let(:object) do
+    Mutant::Scope.new(
+      expression: instance_double(Mutant::Expression),
+      raw:        raw_scope
+    )
+  end
+
   describe '#unqualified_name' do
     subject { object.unqualified_name }
 
-    let(:object) do
-      Mutant::Scope.new(
-        expression: instance_double(Mutant::Expression),
-        raw:        raw_scope
-      )
-    end
-
     context 'with top level constant name' do
       let(:raw_scope) { TestApp }
 
@@ -31,4 +31,53 @@
       it_behaves_like 'an idempotent method'
     end
   end
+
+  describe '#match_expressions' do
+    subject { object.match_expressions }
+
+    def recursive(scope_name)
+      Mutant::Expression::Namespace::Recursive.new(scope_name:)
+    end
+
+    context 'with top level constant name' do
+      let(:raw_scope) { TestApp }
+
+      it 'returns single recursive expression' do
+        is_expected.to eql([recursive('TestApp')])
+      end
+
+      it_behaves_like 'an idempotent method'
+    end
+
+    context 'with two level constant name' do
+      let(:raw_scope) { TestApp::Literal }
+
+      it 'returns expressions from most to least specific' do
+        is_expected.to eql(
+          [
+            recursive('TestApp::Literal'),
+            recursive('TestApp')
+          ]
+        )
+      end
+
+      it_behaves_like 'an idempotent method'
+    end
+
+    context 'with three level constant name' do
+      let(:raw_scope) { double('scope', name: 'TestApp::Literal::Deep') }
+
+      it 'returns expressions from most to least specific' do
+        is_expected.to eql(
+          [
+            recursive('TestApp::Literal::Deep'),
+            recursive('TestApp::Literal'),
+            recursive('TestApp')
+          ]
+        )
+      end
+
+      it_behaves_like 'an idempotent method'
+    end
+  end
 end
diff --git a/ruby/spec/unit/mutant/world_spec.rb b/ruby/spec/unit/mutant/world_spec.rb
index 80fb77938..acff1381a 100644
--- a/ruby/spec/unit/mutant/world_spec.rb
+++ b/ruby/spec/unit/mutant/world_spec.rb
@@ -81,7 +81,9 @@ def apply
     end
 
     before do
-      allow(open3).to receive_messages(capture3: [stdout, stderr, process_status])
+      allow(open3).to receive(:capture3)
+        .with(*command, binmode: true)
+        .and_return([stdout, stderr, process_status])
     end
 
     context 'when process exists successful' do