Hi.

Coming from PHP, specifically PHP 7.1 where the concept of union types are supported.

Currently, where a method may return more than 1 type - commonly a boolean false and some other type, this is documented as mixed.

For example:

mixed strpos ( string $haystack , mixed $needle [, int $offset = 0 ] )

This is rendered from this docbook snippet:

  <methodsynopsis>
   <type>mixed</type><methodname>strpos</methodname>
   <methodparam><type>string</type><parameter>haystack</parameter></methodparam>
   <methodparam><type>mixed</type><parameter>needle</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>offset</parameter><initializer>0</initializer></methodparam>
  </methodsynopsis>

The result for this function will either be a boolean false or a positive integer, but as a single <type> is allowed, <type>mixed</type> is what we are currently using.

It would be useful to enhance docbook to allow for multiple types for <methodsynopsis> and <methodparam>.

So, my proposal is a new tag of <types> that allows for 2 or more <type>.

methodsynopsis ::=
	Sequence of:
		Zero or more of:
			modifier
		Optionally one of:
			type
			types
				Two or more of:
					type
			void
		methodname
		One of:
			One or more of:
				group (db.group.methodparam)
				methodparam
			void
		Zero or more of:
			exceptionname
		Zero or more of:
			modifier

and

methodparam ::=
	Sequence of:
		Zero or more of:
			modifier
		Optionally one of
			type
			types
				Two or more of:
					type
		One of:
			Sequence of:
				Zero or more of:
					modifier
				parameter
				initializer?
			funcparams
		Zero or more of:
			modifier

If there are other places where a <type> is defined, these may also need to be expanded to allow for multiple types.

If there is no way to require 2 or more of a child tag, then 1 or more would be fine, and would make <type>int</type> the same as <types><type>int</type></types>.

I'm happy to make a proper pull request if pointed in the right direction.