Get the casing of `value`.
function casing(value) {
  var head = exact(value.charAt(0))
  var rest = value.slice(1)

  if (!rest) {
    return head
  }

  rest = exact(rest)

  if (head === rest) {
    return head
  }

  if (head === 'u' && rest === 'l') {
    return 's'
  }

  return null
}