diff --git a/test/ruby/sentgen.rb b/test/ruby/sentgen.rb
index 29388e91d5..94cffdad7a 100644
--- a/test/ruby/sentgen.rb
+++ b/test/ruby/sentgen.rb
@@ -1,10 +1,202 @@
 # sentence generator
 
 class SentGen
+  def SentGen.each_tree(syntax, sym, limit, &b)
+    SentGen.new(syntax).each_tree(sym, limit, &b)
+  end
+
+  def SentGen.each_string(syntax, sym, limit, &b)
+    SentGen.new(syntax).each_string(sym, limit, &b)
+  end
+
   def initialize(syntax)
     @syntax = syntax
   end
 
+  def self.expand_syntax(syntax)
+    syntax = remove_underivable_rules(syntax)
+    syntax = expand_justempty_rules(syntax)
+    syntax = remove_emptyable_rules(syntax)
+    syntax = expand_channel_rules(syntax)
+
+    syntax = expand_noalt_rules(syntax)
+    syntax = reorder_rules(syntax)
+  end
+
+  def self.remove_underivable_rules(syntax)
+    deribable_syms = {}
+    changed = true
+    while changed
+      changed = false
+      syntax.each {|sym, rules|
+        next if deribable_syms[sym]
+        rules.each {|rhs|
+          if rhs.all? {|e| String === e || deribable_syms[e] }
+            deribable_syms[sym] = true
+            changed = true
+            break
+          end
+        }
+      }
+    end
+    result = {}
+    syntax.each {|sym, rules|
+      next if !deribable_syms[sym]
+      rules2 = []
+      rules.each {|rhs|
+        rules2 << rhs if rhs.all? {|e| String === e || deribable_syms[e] }
+      }
+      result[sym] = rules2.uniq
+    }
+    result
+  end
+
+  def self.expand_justempty_rules(syntax)
+    justempty_syms = {}
+    changed = true
+    while changed
+      changed = false
+      syntax.each {|sym, rules|
+        next if justempty_syms[sym]
+        if rules.all? {|rhs| rhs.all? {|e| justempty_syms[e] } }
+          justempty_syms[sym] = true
+          changed = true
+        end
+      }
+    end
+    result = {}
+    syntax.each {|sym, rules|
+      result[sym] = rules.map {|rhs| rhs.reject {|e| justempty_syms[e] } }.uniq
+    }
+    result
+  end
+
+  def self.expand_emptyable_syms(rhs, emptyable_syms)
+    if rhs.empty?
+    elsif rhs.length == 1
+      if emptyable_syms[rhs[0]]
+        yield rhs
+        yield []
+      else
+        yield rhs
+      end
+    else
+      butfirst = rhs[1..-1]
+      if emptyable_syms[rhs[0]]
+        expand_emptyable_syms(butfirst, emptyable_syms) {|rhs2| 
+          yield [rhs[0]] + rhs2
+          yield rhs2
+        }
+      else
+        expand_emptyable_syms(butfirst, emptyable_syms) {|rhs2|
+          yield [rhs[0]] + rhs2
+        }
+      end
+    end
+  end
+
+  def self.remove_emptyable_rules(syntax)
+    emptyable_syms = {}
+    changed = true
+    while changed
+      changed = false
+      syntax.each {|sym, rules|
+        next if emptyable_syms[sym]
+        rules.each {|rhs|
+          if rhs.all? {|e| emptyable_syms[e] }
+            emptyable_syms[sym] = true
+            changed = true
+            break
+          end
+        }
+      }
+    end
+    result = {}
+    syntax.each {|sym, rules|
+      rules2 = []
+      rules.each {|rhs|
+        expand_emptyable_syms(rhs, emptyable_syms) {|rhs2|
+          rules2 << rhs2
+        }
+      }
+      result[sym] = rules2.uniq
+    }
+    result
+  end
+
+  def self.expand_channel_rules(syntax)
+    channel_rules = {}
+    syntax.each {|sym, rules|
+      channel_rules[sym] = {sym=>true}
+      rules.each {|rhs|
+        if rhs.length == 1 && Symbol === rhs[0]
+          channel_rules[sym][rhs[0]] = true
+        end
+      }
+    }
+    changed = true
+    while changed
+      changed = false 
+      channel_rules.each {|sym, set|
+        n1 = set.size
+        set.keys.each {|s|
+          set.update(channel_rules[s])
+        }
+        n2 = set.size
+        changed = true if n1 < n2
+      }
+    end
+    result = {}
+    syntax.each {|sym, rules|
+      rules2 = []
+      channel_rules[sym].each_key {|s|
+        syntax[s].each {|rhs|
+          unless rhs.length == 1 && Symbol === rhs[0]
+            rules2 << rhs
+          end
+        }
+      }
+      result[sym] = rules2.uniq
+    }
+    result
+  end
+
+  def self.expand_noalt_rules(syntax)
+    noalt_syms = {}
+    syntax.each {|sym, rules|
+      if rules.length == 1
+        noalt_syms[sym] = true
+      end
+    }
+    result = {}
+    syntax.each {|sym, rules|
+      rules2 = []
+      rules.each {|rhs|
+        rhs2 = []
+        rhs.each {|e|
+          if noalt_syms[e]
+            rhs2.concat syntax[e][0]
+          else
+            rhs2 << e
+          end
+        }
+        rules2 << rhs2
+      }
+      result[sym] = rules2.uniq
+    }
+    result
+  end
+
+  def self.reorder_rules(syntax)
+    result = {}
+    syntax.each {|sym, rules|
+      result[sym] = rules.sort_by {|rhs|
+        [rhs.find_all {|e| Symbol === e }.length, rhs.length]
+      }
+    }
+    result
+  end
+
   def each_tree(sym, limit)
     generate_from_sym(sym, limit) {|_, tree|
       yield tree
@@ -25,6 +217,7 @@ class SentGen
       yield limit, sym
     else
       rules = @syntax[sym]
+      raise "undefined rule: #{sym}" if !rules
       rules.each {|rhs|
         if rhs.length == 1 || rules.length == 1
           limit1 = limit
diff --git a/test/ruby/test_assignment.rb b/test/ruby/test_assignment.rb
index 1a07575f94..33c0be027c 100644
--- a/test/ruby/test_assignment.rb
+++ b/test/ruby/test_assignment.rb
@@ -665,7 +665,8 @@ class TestAssignmentGen < Test::Unit::TestCase
   end
 
   def test_assignment
-    SentGen.new(Syntax).each_tree(:xassign, 3) {|assign|
+    syntax = SentGen.expand_syntax(Syntax)
+    SentGen.each_tree(syntax, :xassign, 3) {|assign|
       assign[0] = rename_var(assign[0])
       sent = [assign].join('')
       bruby = do_assign(assign).to_a.sort