gistfile1.rb

class Rake::Task

  def overwrite(&block)
    abandon
    enhance(&block)
  end

  def abandon
    @actions.clear
    prerequisites.clear
  end

end

# Usage:
#
# 1) Undefine task "test"
# >> Rake::Task[:test].abandon
#
# 2) Overwrite task "test"
# >> Rake::task[:test].overwrite { ... }