function! Test_WriteFile()
  let f = tempname()
  call writefile(["over","written"], f, "b")
  call writefile(["hello","world"], f, "b")
  call writefile(["!", "good"], f, "a")
  call writefile(["morning"], f, "ab")
  call writefile(["", "vimmers"], f, "ab")
  let l = readfile(f)
  call assert_equal("hello", l[0])
  call assert_equal("world!", l[1])
  call assert_equal("good", l[2])
  call assert_equal("morning", l[3])
  call assert_equal("vimmers", l[4])
  call delete(f)
endfunction