code: 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
  }
docs: Get the casing of `value`.