diff --git a/.rspec b/.rspec
new file mode 100644
index 0000000..5be63fc
--- /dev/null
+++ b/.rspec
@@ -0,0 +1,2 @@
+--require spec_helper
+--format documentation
diff --git a/Gemfile b/Gemfile
new file mode 100644
index 0000000..747c9a6
--- /dev/null
+++ b/Gemfile
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+group :development, :test do
+  gem "rspec", "~> 3.13"
+  gem "rubocop", require: false
+  gem "rake"
+end
diff --git a/Gemfile.lock b/Gemfile.lock
new file mode 100644
index 0000000..0ed9a5b
--- /dev/null
+++ b/Gemfile.lock
@@ -0,0 +1,60 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    ast (2.4.3)
+    diff-lcs (1.6.2)
+    json (2.13.2)
+    language_server-protocol (3.17.0.5)
+    lint_roller (1.1.0)
+    parallel (1.27.0)
+    parser (3.3.9.0)
+      ast (~> 2.4.1)
+      racc
+    prism (1.4.0)
+    racc (1.8.1)
+    rainbow (3.1.1)
+    rake (13.3.0)
+    regexp_parser (2.11.2)
+    rspec (3.13.1)
+      rspec-core (~> 3.13.0)
+      rspec-expectations (~> 3.13.0)
+      rspec-mocks (~> 3.13.0)
+    rspec-core (3.13.5)
+      rspec-support (~> 3.13.0)
+    rspec-expectations (3.13.5)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.13.0)
+    rspec-mocks (3.13.5)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.13.0)
+    rspec-support (3.13.5)
+    rubocop (1.80.0)
+      json (~> 2.3)
+      language_server-protocol (~> 3.17.0.2)
+      lint_roller (~> 1.1.0)
+      parallel (~> 1.10)
+      parser (>= 3.3.0.2)
+      rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 2.9.3, < 3.0)
+      rubocop-ast (>= 1.46.0, < 2.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 2.4.0, < 4.0)
+    rubocop-ast (1.46.0)
+      parser (>= 3.3.7.2)
+      prism (~> 1.4)
+    ruby-progressbar (1.13.0)
+    unicode-display_width (3.1.5)
+      unicode-emoji (~> 4.0, >= 4.0.4)
+    unicode-emoji (4.0.4)
+
+PLATFORMS
+  ruby
+  x86_64-linux
+
+DEPENDENCIES
+  rake
+  rspec (~> 3.13)
+  rubocop
+
+BUNDLED WITH
+   2.6.2
diff --git a/README.md b/README.md
index ebf5020..2e998e2 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,19 @@
-#Suchimer
-suchi's sucheme by ruby (2010/12/21-)
-Sorry, test. Please ignore this.
+## Suchemer
+sucheme in Ruby (original: 2010/12/21-). This repo now includes a simple Lisp-style Cons implementation.
+
+### Cons usage
+```ruby
+require "suchemer/cons"
+include Suchemer
+
+lst = Cons.list(1,2,3)  # => (1 2 3)
+lst.to_a                 # => [1,2,3]
+lst[1]                   # => 2
+lst.reverse              # => (3 2 1)
+lst.append([4,5])        # => (1 2 3 4 5)
+
+Cons.new(1, 2)           # => (1 . 2)
+```
+
+See specs under `spec/` for more.
 
diff --git a/lib/suchemer/cons.rb b/lib/suchemer/cons.rb
index efe6b64..a9ade18 100644
--- a/lib/suchemer/cons.rb
+++ b/lib/suchemer/cons.rb
@@ -1,8 +1,192 @@
 module Suchemer
   class Cons
+    # A simple Lisp-style cons cell with basic list operations.
+    # - A proper list is a chain of Cons ending in nil.
+    # - A dotted pair is a Cons whose cdr is neither Cons nor nil.
+
+    include Enumerable
+
     attr_accessor :car, :cdr
-    def initialize(car, cdr) 
-      @car, @cdr = car, cdr
+
+    def initialize(car = nil, cdr = nil)
+      @car = car
+      @cdr = cdr
     end
+
+    # Build a proper list from given elements.
+    def self.list(*elements)
+      from_array(elements)
+    end
+
+    # Build a proper list from an Array or any Enumerable.
+    def self.from_array(enum)
+      arr = enum.respond_to?(:to_a) ? enum.to_a : Array(enum)
+      node = nil
+      arr.reverse_each { |e| node = new(e, node) }
+      node
+    end
+
+    class << self
+      alias from_enum from_array
+      alias pair new
+    end
+
+    # Iterate over car values of a proper list. Stops before a dotted tail if any.
+    def each
+      return enum_for(:each) unless block_given?
+      node = self
+      while node.is_a?(Cons)
+        yield node.car
+        node = node.cdr
+      end
+      self
+    end
+
+    # Convert to a Ruby Array (drops a dotted tail if present).
+    def to_a
+      map { |x| x }
+    end
+
+    # Number of elements in the proper part of the list.
+    def size
+      count
+    end
+    alias length size
+
+    # Whether this is a proper list (cdr chain ends with nil).
+    def proper_list?
+      node = self
+      while node.is_a?(Cons)
+        node = node.cdr
+      end
+      node.nil?
+    end
+
+    # Whether this cons is a dotted pair (cdr is not Cons and not nil).
+    def dotted_pair?
+      !(@cdr.is_a?(Cons) || @cdr.nil?)
+    end
+
+    # Array-like element access. Supports negative indices via conversion.
+    def [](index)
+      unless index.is_a?(Integer)
+        raise TypeError, "index must be Integer"
+      end
+      return to_a[index] if index.negative?
+
+      i = 0
+      node = self
+      while node.is_a?(Cons) && i < index
+        node = node.cdr
+        i += 1
+      end
+      return nil unless node.is_a?(Cons)
+      node.car
+    end
+    alias at []
+
+    # Return the last element of a proper list. For a dotted pair chain,
+    # returns the cdr of the final cons.
+    def last
+      node = self
+      prev = nil
+      while node.is_a?(Cons)
+        prev = node
+        node = node.cdr
+      end
+      # node is now either nil (proper list) or dotted tail
+      if node.nil?
+        prev&.car
+      else
+        # dotted tail value
+        node
+      end
+    end
+
+    # Return a new proper list with same elements in reverse order.
+    # Raises ArgumentError if not a proper list.
+    def reverse
+      raise ArgumentError, "reverse requires a proper list" unless proper_list?
+      acc = nil
+      node = self
+      while node.is_a?(Cons)
+        acc = Cons.new(node.car, acc)
+        node = node.cdr
+      end
+      acc
+    end
+
+    # Append another list/array to this list, returning a new list.
+    # Accepts: Cons, Array, or nil. Requires this to be a proper list.
+    def append(other)
+      raise ArgumentError, "append requires a proper list on the left-hand side" unless proper_list?
+
+      rhs =
+        case other
+        when nil
+          nil
+        when Cons
+          other
+        when Array
+          Cons.from_array(other)
+        else
+          raise ArgumentError, "append accepts Cons, Array, or nil"
+        end
+
+      # Copy this list and attach rhs at the end.
+      return rhs if self.nil?
+      # copy head
+      head = Cons.new(@car, nil)
+      dst = head
+      src = @cdr
+      while src.is_a?(Cons)
+        dst.cdr = Cons.new(src.car, nil)
+        dst = dst.cdr
+        src = src.cdr
+      end
+      dst.cdr = rhs
+      head
+    end
+
+    # Structural equality. Compares element-wise for proper lists.
+    # Also supports comparison with Arrays.
+    def ==(other)
+      case other
+      when Cons
+        a = self
+        b = other
+        while a.is_a?(Cons) && b.is_a?(Cons)
+          return false unless a.car == b.car
+          a = a.cdr
+          b = b.cdr
+        end
+        a.nil? && b.nil?
+      when Array
+        to_a == other
+      else
+        false
+      end
+    end
+
+    # Pretty print like Lisp: (1 2 3) or (1 . 2)
+    def to_s
+      parts = []
+      node = self
+      while node.is_a?(Cons)
+        parts << node.car.inspect
+        node = node.cdr
+      end
+      if node.nil?
+        "(" + parts.join(" ") + ")"
+      else
+        # dotted tail
+        "(" + parts.join(" ") + " . #{node.inspect})"
+      end
+    end
+    alias inspect to_s
+
+    # Convenience aliases for lispy naming
+    alias head car
+    alias tail cdr
   end
 end
diff --git a/spec/cons_spec.rb b/spec/cons_spec.rb
new file mode 100644
index 0000000..e6830ab
--- /dev/null
+++ b/spec/cons_spec.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+module Suchemer
+  RSpec.describe Cons do
+    describe ".list/.from_array" do
+      it "builds a proper list" do
+        lst = Cons.list(1, 2, 3)
+        expect(lst.to_a).to eq [1, 2, 3]
+        expect(lst.proper_list?).to be true
+      end
+    end
+
+    describe "Enumerable" do
+      it "iterates over car values" do
+        expect(Cons.list(1,2,3).map { |x| x * 2 }).to eq [2,4,6]
+      end
+    end
+
+    describe "indexing" do
+      it "supports positive and negative indices" do
+        lst = Cons.list(1,2,3)
+        expect(lst[0]).to eq 1
+        expect(lst[1]).to eq 2
+        expect(lst[-1]).to eq 3
+      end
+    end
+
+    describe "reverse" do
+      it "reverses a proper list" do
+        expect(Cons.list(1,2,3).reverse.to_a).to eq [3,2,1]
+      end
+
+      it "raises on dotted list" do
+        dotted = Cons.new(1, 2)
+        expect { dotted.reverse }.to raise_error(ArgumentError)
+      end
+    end
+
+    describe "append" do
+      it "appends array or cons" do
+        expect(Cons.list(1,2).append([3,4]).to_a).to eq [1,2,3,4]
+        expect(Cons.list(1,2).append(Cons.list(3,4)).to_a).to eq [1,2,3,4]
+      end
+    end
+
+    describe "equality" do
+      it "compares structurally" do
+        expect(Cons.list(1,2,3)).to eq Cons.new(1, Cons.new(2, Cons.new(3, nil)))
+        expect(Cons.list(1,2,3)).to eq [1,2,3]
+      end
+    end
+
+    describe "printing" do
+      it "prints proper and dotted lists" do
+        expect(Cons.list(1,2,3).to_s).to eq "(1 2 3)"
+        expect(Cons.new(1, 2).to_s).to eq "(1 . 2)"
+      end
+    end
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
new file mode 100644
index 0000000..a598e84
--- /dev/null
+++ b/spec/spec_helper.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require "rspec"
+require_relative "../lib/suchemer/cons"
+
+RSpec.configure do |config|
+  config.expect_with :rspec do |c|
+    c.syntax = :expect
+  end
+
+  config.mock_with :rspec do |m|
+    m.verify_partial_doubles = true
+  end
+end