Execute (Setup):
  Save g:colors_name, &background
  colo seoul256

  function! s:assert_on()
    Assert exists('#limelight'), 'limelight should be on'
  endfunction
  function! s:assert_off()
    Assert !exists('#limelight'), 'limelight should be off'
  endfunction
  command! AssertOn  call s:assert_on()
  command! AssertOff call s:assert_off()

Before:
  unlet! g:limelight_default_coefficient
       \ g:limelight_conceal_guifg g:limelight_conceal_ctermfg

Execute (Invalid argument type):
  Limelight 'hello'
  AssertOff

  Limelight {}
  AssertOff

  Limelight []
  AssertOff

Execute (Invalid argument range):
  Limelight 1.2
  AssertOff

  Limelight 2
  AssertOff

  Limelight -0.2
  AssertOff

  Limelight -1
  AssertOff

Execute (Limelight / Limelight!):
  Limelight
  AssertOn

  Limelight
  AssertOn

  Limelight!
  AssertOff

  Limelight!
  AssertOff

  Limelight 0.5
  AssertOn

  Limelight!
  AssertOff

Execute (Limelight!!):
  Limelight!!
  AssertOn

  Limelight!!
  AssertOff

  Limelight!! 2.0
  AssertOff

  Limelight!! 0.9
  AssertOn

  Limelight!! 0.9
  AssertOff

Execute (g:limelight_default_coefficient):
  let g:limelight_default_coefficient = 2.0
  Limelight
  AssertOff

  let g:limelight_default_coefficient = -1.0
  Limelight
  AssertOff

Execute (Unsupported terminal):
  colo default
  let &t_Co = 16

  Limelight
  AssertOff

Execute (Unsupported. but g:limelight_conceal_???fg defined):
  if has('gui_running')
    colo default
    let g:limelight_conceal_guifg = '#cccccc'
    Limelight
    AssertOn
    Limelight!
    AssertOff
  else
    if exists('&termguicolors')
      set notermguicolors
    endif
    colo default
    let &t_Co = 256

    " Cannot enable it without g:limelight_conceal_ctermfg
    Limelight
    AssertOff

    let g:limelight_conceal_ctermfg = 240
    Limelight
    AssertOn
    Limelight!
    AssertOff

    let g:limelight_conceal_ctermfg = 'red'
    Limelight
    AssertOn
    Limelight!
    AssertOff
  endif

  " However, you're still not allowed to pass argument to Limelight command
  redir => msg
  Limelight 0.5
  redir END
  AssertOff
  Assert msg !~ 'g:limelight_conceal_', 'Invalid message: '.msg

Execute (TODO: Color interpolation):
  Assert 0, 'TODO'

Execute (Cleanup):
  delc AssertOn
  delc AssertOff
  unlet! msg
  Restore
  silent! execute 'colo '.g:colors_name