diff --git a/test/ruby/sentence.rb b/test/ruby/sentence.rb
index 3697fbea10..3eaaf9c11a 100644
--- a/test/ruby/sentence.rb
+++ b/test/ruby/sentence.rb
@@ -1,30 +1,204 @@
-# sentence
+# == sentence library
+#
+# = Features
+#
+# * syntax based sentences generation
+# * sentence operations such as substitution.
+#
+# = Example
+#
+# Some arithmetic expressions using "+", "-", "*" and "/" are generated as follows.
+#
+#  require 'sentence'
+#  Sentence.each({
+#    :exp => [["num"],
+#             [:exp, "+", :exp],
+#             [:exp, "-", :exp],
+#             [:exp, "*", :exp],
+#             [:exp, "/", :exp]]
+#  }, :exp, 2) {|sent| p sent }
+#  #=>
+#  #<Sentence: "num">
+#  #<Sentence: ("num") "+" ("num")>
+#  #<Sentence: ("num") "+" (("num") "+" ("num"))>
+#  #<Sentence: ("num") "+" (("num") "-" ("num"))>
+#  #<Sentence: ("num") "+" (("num") "*" ("num"))>
+#  #<Sentence: ("num") "+" (("num") "/" ("num"))>
+#  #<Sentence: (("num") "+" ("num")) "+" ("num")>
+#  ...
+#
+# Sentence.each takes 3 arguments.
+# The first argument is the syntax for the expressions.
+# The second argument, :exp, is a generating nonterminal.
+# The third argument, 2, limits derivation to restrict results finitely.
+#
+# Some arithmetic expressions including parenthesis can be generated as follows.
+# 
+#  synax = {
+#    :factor => [["n"],
+#                ["(", :exp, ")"]],
+#    :term => [[:factor],
+#              [:term, "*", :factor],
+#              [:term, "/", :factor]],
+#    :exp => [[:term],
+#             [:exp, "+", :term],
+#             [:exp, "-", :term]]
+#  }
+#  Sentence.each(syntax, :exp, 2) {|sent| p sent }
+#  #=>
+#  #<Sentence: (("n"))>
+#  #<Sentence: (("(" ((("n"))) ")"))>
+#  #<Sentence: (("(" ((("(" ((("n"))) ")"))) ")"))>
+#  #<Sentence: (("(" (((("n")) "*" ("n"))) ")"))>
+#  #<Sentence: (("(" (((("n")) "/" ("n"))) ")"))>
+#  #<Sentence: (("(" (((("n"))) "+" (("n"))) ")"))>
+#  #<Sentence: (("(" (((("n"))) "-" (("n"))) ")"))>
+#  #<Sentence: ((("n")) "*" ("n"))>
+#  #<Sentence: ((("n")) "*" ("(" ((("n"))) ")"))>
+#  ...
+#
+#  Sentence#to_s can be used to concatenate strings
+#  in a sentence:
+#
+#  Sentence.each(syntax, :exp, 2) {|sent| p sent.to_s }
+#  #=>
+#  "n"
+#  "(n)"
+#  "((n))"
+#  "(n*n)"
+#  "(n/n)"
+#  "(n+n)"
+#  "(n-n)"
+#  "n*n"
+#  "n*(n)"
+#  ...
+#
 
+# Sentence class represents a tree with string leaves.
+#
 class Sentence
+  # _ary_ represents a tree.
+  # It should be a possibly nested array which contains strings.
+  #
+  # Note that _ary_ is not copied.
+  # Don't modify _ary_ after the sentence object is instantiated.
+  #
+  #  Sentence.new(["a", "pen"])
+  #  #<Sentence: "a" "pen">
+  #
+  #  Sentence.new(["I", "have", ["a", "pen"]])
+  #  #<Sentence: "I" "have" ("a" "pen")>
+  #
   def initialize(ary)
     @sent = ary
   end
 
+  # returns a string which is concatenation of all strings.
+  # No separator is used.
+  #
+  #  Sentence.new(["2", "+", "3"]).to_s
+  #  "2+3"
+  #
+  #  Sentence.new(["2", "+", ["3", "*", "5"]]).to_s
+  #  "2+3*5"
+  #
   def to_s
     @sent.join('')
   end
 
+  # returns a string which is concatenation of all strings separated by _sep_.
+  # If _sep_ is not given, single space is used.
+  #
+  #  Sentence.new(["I", "have", ["a", "pen"]]).join
+  #  "I have a pen"
+  #
+  #  Sentence.new(["I", "have", ["a", "pen"]]).join("/")
+  #  "I/have/a/pen"
+  #
+  #  Sentence.new(["a", [], "b"]).join("/")
+  #  "a/b"
+  #
+  def join(sep=' ')
+    @sent.flatten.join(sep)
+  end
+
+  # returns a tree as a nested array.
+  #
+  # Note that the result is not copied.
+  # Don't modify the result.
+  #
+  #  Sentence.new([["foo", "bar"], "baz"]).to_a
+  #  #=> [["foo", "bar"], "baz"]
+  #
   def to_a
     @sent
   end
 
+  # returns <i>i</i>th element as a sentence or string.
+  #
+  #  s = Sentence.new([["foo", "bar"], "baz"])
+  #  s    #=> #<Sentence: ("foo" "bar") "baz">
+  #  s[0] #=> #<Sentence: "foo" "bar">
+  #  s[1] #=> "baz"
+  #
   def [](i)
-    Sentence.new(@sent[i])
+    e = @sent[i]
+    e.respond_to?(:to_ary) ? Sentence.new(e) : e
+  end
+
+  # returns the number of top level elements.
+  #
+  #  Sentence.new(%w[foo bar]).length   
+  #  #=> 2
+  #
+  #  Sentence.new([%w[2 * 7], "+", %w[3 * 5]]).length                         
+  #  #=> 3
+  #
+  def length
+    @sent.length
   end
 
   def inspect
-    "#<#{self.class}: #{@sent.inspect}>"
+    "#<#{self.class}: #{inner_inspect(@sent, '')}>"
   end
 
+  # :stopdoc:
+  def inner_inspect(ary, r)
+    first = true
+    ary.each {|obj|
+      r << ' ' if !first
+      first = false
+      if obj.respond_to? :to_ary
+        r << '('
+        inner_inspect(obj, r)
+        r << ')'
+      else
+        r << obj.inspect
+      end
+    }
+    r
+  end
+  # :startdoc:
+
+  # returns new sentence object which
+  # _target_ is substituted by the block.
+  #
+  # Sentence#subst invokes <tt>_target_ === _string_</tt> for each
+  # string in the sentence.
+  # The strings which === returns true are substituted by the block.
+  # The block is invoked with the substituting string.
+  #
+  #  Sentence.new(%w[2 + 3]).subst("+") { "*" }
+  #  #<Sentence: "2" "*" "3">
+  #
+  #  Sentence.new(%w[2 + 3]).subst(/\A\d+\z/) {|s| ((s.to_i)*2).to_s }
+  #  #=> #<Sentence: "4" "+" "6">
+  #
   def subst(target, &b)
     Sentence.new(subst_rec(@sent, target, &b))
   end
 
+  # :stopdoc:
   def subst_rec(obj, target, &b)
     if obj.respond_to? :to_ary
       a = []
@@ -36,19 +210,25 @@ class Sentence
       obj
     end
   end
+  # :startdoc:
 
+  # find a subsentence and return it.
+  # The block is invoked for each subsentence in preorder manner.
+  # The first subsentence which the block returns true is returned.
+  #
+  #  Sentence.new([%w[2 * 7], "+", %w[3 * 5]]).find_subtree {|s| s[1] == "*" }
+  #  #=> #<Sentence: "2" "*" "7">
+  #
   def find_subtree(&b)
-    if r = find_subtree_rec(@sent, &b)
-      Sentence.new(r)
-    else
-      nil
-    end
+    find_subtree_rec(@sent, &b)
   end
 
+  # :stopdoc:
   def find_subtree_rec(obj, &b)
     if obj.respond_to? :to_ary
-      if b.call obj
-        return obj
+      s = Sentence.new(obj)
+      if b.call s
+        return s
       else
         obj.each {|e|
           r = find_subtree_rec(e, &b)
@@ -58,15 +238,33 @@ class Sentence
     end
     nil
   end
+  # :startdoc:
 
+  # returns a new sentence object which expands according to the condition
+  # given by the block.
+  #
+  # The block is invoked for each subsentence.
+  # The subsentences which the block returns true are
+  # expanded into parent.
+  #
+  #  s = Sentence.new([%w[2 * 7], "+", %w[3 * 5]])
+  #  #=> #<Sentence: ("2" "*" "7") "+" ("3" "*" "5")>
+  #
+  #  s.expand { true }
+  #  #=> #<Sentence: "2" "*" "7" "+" "3" "*" "5">
+  #
+  #  s.expand {|s| s[0] == "3" }
+  #  #=> #<Sentence: (("2" "*" "7") "+" "3" "*" "5")>
+  #
   def expand(&b)
     Sentence.new(expand_rec(@sent, &b))
   end
 
+  # :stopdoc:
   def expand_rec(obj, r=[], &b)
-    #puts "#{obj.inspect}\t\t#{r.inspect}"
     if obj.respond_to? :to_ary
-      if b.call obj
+      s = Sentence.new(obj)
+      if b.call s
         obj.each {|o|
           expand_rec(o, r, &b)
         }
@@ -82,13 +280,80 @@ class Sentence
     end
     r
   end
+  # :startdoc:
 
+  # Sentence.each generates sentences
+  # by deriving the start symbol _sym_ using _syntax_.
+  # The derivation is restricted by an positive integer _limit_ to
+  # avoid infinite generation.
+  #
+  # Sentence.each yields the block with a generated sentence.
+  #
+  #  Sentence.each({
+  #    :exp => [["n"],
+  #             [:exp, "+", :exp],
+  #             [:exp, "*", :exp]]
+  #    }, :exp, 1) {|sent| p sent }
+  #  #=>
+  #  #<Sentence: "n">
+  #  #<Sentence: ("n") "+" ("n")>
+  #  #<Sentence: ("n") "*" ("n")>
+  #  
+  #  Sentence.each({
+  #    :exp => [["n"],
+  #             [:exp, "+", :exp],
+  #             [:exp, "*", :exp]]
+  #    }, :exp, 2) {|sent| p sent }
+  #  #=>
+  #  #<Sentence: "n">
+  #  #<Sentence: ("n") "+" ("n")>
+  #  #<Sentence: ("n") "+" (("n") "+" ("n"))>
+  #  #<Sentence: ("n") "+" (("n") "*" ("n"))>
+  #  #<Sentence: (("n") "+" ("n")) "+" ("n")>
+  #  #<Sentence: (("n") "*" ("n")) "+" ("n")>
+  #  #<Sentence: ("n") "*" ("n")>
+  #  #<Sentence: ("n") "*" (("n") "+" ("n"))>
+  #  #<Sentence: ("n") "*" (("n") "*" ("n"))>
+  #  #<Sentence: (("n") "+" ("n")) "*" ("n")>
+  #  #<Sentence: (("n") "*" ("n")) "*" ("n")>
+  #
   def Sentence.each(syntax, sym, limit)
     Gen.new(syntax).each_tree(sym, limit) {|tree|
       yield Sentence.new(tree)
     }
   end
 
+  # Sentence.expand_syntax returns an expanded syntax:
+  # * no underivable rule
+  # * no rule which derives only to empty sequence indirectly
+  # * no rule which is derivable to empty and non-empty
+  # * no channel rule
+  #
+  # Note that the rules which can derive empty and non-empty
+  # sequences are modified to derive only non-empty sequences.
+  #
+  #  Sentence.expand_syntax({
+  #    :underivable => [[:underivable]],
+  #    :just_empty1 => [[]],
+  #    :just_empty2 => [[:just_empty1, :just_empty1]],
+  #    :empty_or_not => [[], ["foo"]],
+  #    :empty_or_not_2 => [[:empty_or_not, :empty_or_not]],
+  #    :data => [["a", "b"], ["c", "d"]],
+  #    :channel => [[:data]],
+  #    })
+  #  #=>
+  #  {:channel=>[["a", "b"], ["c", "d"]],
+  #   :data=>[["a", "b"], ["c", "d"]],
+  #   :empty_or_not=>[["foo"]],
+  #   :empty_or_not_2=>[[], ["foo"], ["foo", "foo"]],
+  #   :just_empty1=>[],
+  #   :just_empty2=>[]}
+  #
+  def Sentence.expand_syntax(syntax)
+    Sentence::Gen.expand_syntax(syntax)
+  end
+
+  # :stopdoc:
   class Gen
     def Gen.each_tree(syntax, sym, limit, &b)
       Gen.new(syntax).each_tree(sym, limit, &b)
@@ -333,6 +598,7 @@ class Sentence
       nil
     end
   end
+  # :startdoc:
 
 end
 
diff --git a/test/ruby/test_assignment.rb b/test/ruby/test_assignment.rb
index 3a1e014714..c6b42d1b36 100644
--- a/test/ruby/test_assignment.rb
+++ b/test/ruby/test_assignment.rb
@@ -658,7 +658,7 @@ class TestAssignmentGen < Test::Unit::TestCase
   end
 
   def test_assignment
-    syntax = Sentence::Gen.expand_syntax(Syntax)
+    syntax = Sentence.expand_syntax(Syntax)
     Sentence.each(syntax, :xassign, 3) {|assign|
       assign, vars = rename_var(assign)
       sent = assign.to_s
diff --git a/test/ruby/test_yield.rb b/test/ruby/test_yield.rb
index 8bc4972bd3..3ae6881b87 100644
--- a/test/ruby/test_yield.rb
+++ b/test/ruby/test_yield.rb
@@ -322,14 +322,14 @@ class TestRubyYieldGen < Test::Unit::TestCase
   end
 
   def test_yield
-    syntax = Sentence::Gen.expand_syntax(Syntax)
+    syntax = Sentence.expand_syntax(Syntax)
     Sentence.each(syntax, :test_proc, 4) {|t|
       check_nofork(t)
     }
   end
 
   def test_yield_lambda
-    syntax = Sentence::Gen.expand_syntax(Syntax)
+    syntax = Sentence.expand_syntax(Syntax)
     Sentence.each(syntax, :test_lambda, 4) {|t|
       check_nofork(t, true)
     }