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diff --git a/dragon/easing.rb b/dragon/easing.rb
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+# coding: utf-8
+# Copyright 2019 DragonRuby LLC
+# MIT License
+# easing.rb has been released under MIT (*only this file*).
+
+module GTK
+  module Easing
+    def self.ease_extended start_tick, current_tick, end_tick, default_before, default_after, *definitions
+      definitions.flatten!
+      definitions = [:identity] if definitions.length == 0
+      duration = end_tick - start_tick
+      elapsed  = current_tick - start_tick
+      y = elapsed.percentage_of(duration).cap_min_max(0, 1)
+
+      definitions.map do |definition|
+        y = Easing.exec_definition(definition, start_tick, duration, y)
+      end
+
+      y
+    end
+
+    def self.ease_spline_extended start_tick, current_tick, end_tick, spline
+      duration = end_tick - start_tick
+      t = (current_tick - start_tick).fdiv duration
+      time_allocation_per_curve = 1.fdiv(spline.length)
+      curve_index, curve_t = t.fdiv(time_allocation_per_curve).let do |spline_t|
+        [spline_t.to_i, spline_t - spline_t.to_i]
+      end
+      Geometry.cubic_bezier curve_t, *spline[curve_index]
+    end
+
+    def self.initial_value *definitions
+      definitions.flatten!
+      return Easing.exec_definition (definitions.value(-1) || :identity), 0, 10, 0
+    end
+
+    def self.final_value *definitions
+      definitions.flatten!
+      return Easing.exec_definition (definitions.value(-1) || :identity), 0, 10, 1.0
+    end
+
+    def self.exec_definition definition, start_tick, duration, x
+      if definition.is_a? Symbol
+        return Easing.send(definition, x).cap_min_max(0, 1)
+      elsif definition.is_a? Proc
+        return definition.call(x, start_tick, duration).cap_min_max(0, 1)
+      end
+
+      raise <<-S
+* ERROR:
+I don't know how to execute easing function with definition #{definition}.
+
+S
+    end
+
+    def self.identity x
+      x
+    end
+
+    def self.flip x
+      1 - x
+    end
+
+    def self.quad x
+      x * x
+    end
+
+    def self.cube x
+      x * x * x
+    end
+
+    def self.quart x
+      x * x * x * x * x
+    end
+
+    def self.quint x
+      x * x * x * x * x * x
+    end
+  end
+end
+
+Easing = GTK::Easing