diff --git a/lib/music.rb b/lib/music.rb
index 2af5f1d..a4aa286 100644
--- a/lib/music.rb
+++ b/lib/music.rb
@@ -21,6 +21,7 @@
 require 'music/key'
 require 'music/interpreter'
 require 'music/score'
+require 'music/gens'
 require 'music/timeline'
 require 'music/smf_writer'
 
@@ -131,12 +132,10 @@ def par(*args)
 =end
   
   # Generate an attribute value from the context.
-  def gen(&fn)
-    lambda { |context| fn[context] }
-  end
+  def gen(&fn) Gen.new(&fn) end
 	
-  # Generate an attribute value from the current time.
-  def env(&fn)
-    gen { |context| fn[context.time] }
-  end
+  # Transform a given attribute value.
+  def tr(&fn) Tr.new(&fn) end
+  
+  def env(&fn) Env.new(&fn) end
 end
diff --git a/lib/music/gens.rb b/lib/music/gens.rb
new file mode 100644
index 0000000..4249b6d
--- /dev/null
+++ b/lib/music/gens.rb
@@ -0,0 +1,23 @@
+module Music
+  class Gen
+    def initialize(&fn)
+      @fn = fn
+    end
+    
+    def apply(name, val, context)
+      @fn.call(context)
+    end
+  end
+  
+  class Tr < Gen
+    def apply(name, val, context)
+      @fn.call(val)
+    end
+  end
+  
+  class Env < Gen
+    def apply(name, val, context)
+      @fn.call(val, context.phase)
+    end
+  end
+end
diff --git a/lib/music/interpreter.rb b/lib/music/interpreter.rb
index 37e4ea3..8c3b71d 100644
--- a/lib/music/interpreter.rb
+++ b/lib/music/interpreter.rb
@@ -6,7 +6,7 @@ def self.eval(music)
         new.eval(music)
       end
       
-      def eval(music, context = Context.default)
+      def eval(music, context = Context.default(music.duration))
         music.eval(self, context)
       end
       
@@ -20,7 +20,9 @@ def eval_section(music, context)
     class Context
       attr_reader :time, :attributes
       
-      def self.default; new(0, {}) end
+      def self.default(duration)
+        new(0, { :section_start => 0, :section_duration => duration})
+      end
       
       def initialize(time, attrs)
         @time, @attributes = time, attrs
@@ -28,6 +30,10 @@ def initialize(time, attrs)
       
       def [](name) attributes[name] end
       
+      def phase
+        (@time - @attributes[:section_start]) / @attributes[:section_duration].to_f
+      end
+      
       def advance(dur)
         self.class.new(time + dur, attributes)
       end
@@ -37,11 +43,17 @@ def push(a0)
         self.class.new(time, a1)
       end
       
-      def accept(attrs)
-        inherited = push(attributes.merge(attrs))
-        push(inherited.attributes.inject({}) do |hsh, (name, val)|
-          hsh.merge(name => val.attr_eval(inherited))
-        end)
+      def accept(a0)
+        names = a0.keys | self.attributes.keys
+        push(names.inject({}) { |a1, name|
+          a1.merge name => case new = self.attributes[name]
+            # If a0 yields a Gen, apply it
+            when Gen: new.apply(name, a0[name], self)
+            # otherwise, inherit the attribute value from a0 if
+            # we haven't defined it
+            else self.attributes[name] || a0[name]
+          end
+        })
       end
     end
   end
diff --git a/lib/music/score.rb b/lib/music/score.rb
index 07358fc..a3d023f 100644
--- a/lib/music/score.rb
+++ b/lib/music/score.rb
@@ -207,7 +207,7 @@ def reverse
       end
       
       def eval(interpreter, c0)
-        c1 = c0.push(attributes)
+        c1 = c0.push(attributes.merge(:section_start => c0.time, :section_duration => duration))
         m  = score.eval(interpreter, c1)
         interpreter.eval_section(m, c0)
       end
diff --git a/spec/gens_spec.rb b/spec/gens_spec.rb
new file mode 100644
index 0000000..d678d67
--- /dev/null
+++ b/spec/gens_spec.rb
@@ -0,0 +1,52 @@
+require File.join( File.dirname(__FILE__), 'spec_helper')
+
+describe Gen do
+  before(:all) do
+    @score = s( n(c4, 1),
+                :amp => gen { |c| 0.5 })
+  end
+  
+  it "should generate values from the given context" do
+    timeline = @score.to_timeline
+    timeline[0].amp.should == 0.5
+  end
+end
+
+describe Tr do
+  before(:all) do
+    @score = s( n(c4, 1, :amp => 0.5),
+                :amp => tr { |amp| amp * 0.5 } )
+  end
+  
+  it "should transform attribute values" do
+    timeline = @score.to_timeline
+    timeline[0].amp.should == 0.25
+  end
+end
+
+describe Env do
+  it "should transform attribute values based on their phase" do
+    score = s( seq(n([c4, e4, g4], 1, :amp => 0.5)),
+                :amp => env { |value, phase| value * Math.sin(phase) } )
+    timeline = score.to_timeline
+    timeline[0].amp.should == 0.0
+    timeline[1].amp.should be_close(0.1636, 0.0001)
+    timeline[2].amp.should be_close(0.3092, 0.0001)
+  end
+  
+  it "can be used to implement various transforms" do
+    score = cresc(2,
+              seq(n([c4, e4, g4], 1, :velocity => 64)))
+    timeline = score.to_timeline
+    timeline[0].velocity.should == 64
+    timeline[1].velocity.should == 85
+    timeline[2].velocity.should == 106
+  end
+  
+  def cresc(factor, score)
+    s(score, :velocity => env { |vel, ph|
+        multiplier = (factor - 1.0) * (1.0 + ph)
+        (vel * multiplier).to_i
+      })
+  end
+end