<p><b>REQUIRED_USE</b></p>

		<p>

			The <c>REQUIRED_USE</c> variable contains a list of assertions that

			must be met by the configuration of USE flags to be valid for this

			ebuild. In order to be matched, a USE flag in a terminal element

			must be enabled (or disabled if it has an exclamation mark prefix).

		</p>

		<p>

			Essentially, <c>REQUIRED_USE</c> is an analogue of <c>DEPEND</c>

			style syntax. For example, to state that some combination is

			forbidden, i.e. "if <c>foo</c> is set, <c>bar</c> must be unset":

		</p>

		<codesample lang="ebuild">

REQUIRED_USE="foo? ( !bar )"

		</codesample>

		<p>

			To state "if <c>foo</c> is set, then at least one of <c>bar</c>,

			<c>baz</c>, and <c>quux</c> must be activated":

		</p>

		<codesample lang="ebuild">

REQUIRED_USE="foo? ( || ( bar baz quux ) )"

		</codesample>

		<p>

			To state "exactly one of <c>foo</c>, <c>bar</c>, or <c>baz</c> must

			be set, but not several":

		</p>

		<codesample lang="ebuild">

REQUIRED_USE="^^ ( foo bar baz )"

		</codesample>

		<p>

			Note that the last relationship is that of an Exclusive OR (XOR).

			While an XOR could be formed from usual <c>DEPEND</c> syntax, a

			specific <c>^^</c> operator has been added for this case.

		</p>

		<important>

			See section

			<uri link="::general-concepts/use-flags/#conflicting-use-flags" />

			for when (and when not) to use <c>REQUIRED_USE</c>.

		</important>

	</li>

	<li>

<p>

	In some exceptional cases, above policy would break reverse USE

	dependencies. To avoid this, the ebuild can specify allowed USE flag

	combinations with <c>REQUIRED_USE</c> (available in EAPI 4). See section

	<uri link="::ebuild-writing/eapi/#eapi=&quot;4&quot;" /> for a description

	of its syntax.

</p>

<p>

	For example, if a package <c>dev-libs/foo</c> can be built with either

	<c>USE="a"</c> or <c>USE="b"</c> but not with both, then preferring one of

	the flags would break packages that depend on either <c>dev-libs/foo[a]</c>

	or <c>dev-libs/foo[b]</c>. Therefore, the ebuild should specify

	<c>REQUIRED_USE="a? ( !b )"</c> in this case.

</p>

<note>

	In order to avoid forcing users to micro-manage flags too much,

	<c>REQUIRED_USE</c> should be used sparingly. Follow the normal policy

	whenever it is possible to do a build that will presumably suit the user's

	needs.

</note>