`or' ... True if one of the argument returns true.
`and' ... True if all of the arguments return true.
-4. Functions which accept not argument.
-
`spam-p' ... True if contents of the message is guessed as spam.
+ Rest arguments are property list which consists
+ following.
+
+ `:register' ... If this value is non-nil,
+ Register according to
+ the classification.
5. A symbol.