gistfile1.rb

x_file = "x.txt"

task :prepare_x do
  bunch_of_files = calculate_all_upstream_files_slowly
  bunch_of_files.each {|filename| file x_file => filename}
end

task :real_x => [x_file]

task :x => [:prepare_x, :real_x]

gistfile2.rb

file "x.txt" => ["a.txt"]
file "x.txt" => ["b.txt"]
file "x.txt" => ["c.txt"]

file "x.txt" do
  output = File.read("a.txt") + File.read("b.txt") + File.read("c.txt")
  File.open("x.txt", "w") {|f| f.write(output) }
end