def included
    def macros = []
    def macroFunctions = []
    def includePaths = [ systemIncludeDir ]

    def setup() {
        included = Mock(IncludeDirectives)
        included.getAllMacros() >> macros
        included.getMacros(_) >> { String name -> macros.findAll {it.name == name} }
        included.hasMacros() >> { !macros.empty }
        included.getAllMacroFunctions() >> macroFunctions

        }

        fileTree.include('*/*included*')
        fileTree.exclude('**/not*')

        expect:
        fileTree.files == [included1, included2] as Set
        assertSetContainsForAllTypes(fileTree, 'subDir/included1', 'subDir2/included2')
        assertVisits(fileTree, ['subDir/included1', 'subDir2/included2'], ['subDir', 'subDir2'])
        fileTree.contains(included1)
        !fileTree.contains(excluded1)

    }

    def "discards duplicate includes"() {
        when:
        sourceFile << """
    #include "test1"
    #include "test1"
    #include "test2"
    #include <test1>
    #include <test1>
    #include <test2>
    #include DEFINED
    #include DEFINED
    #include DEFINED()
    #include DEFINED()
    #include DEFINED(ABC)
    #include DEFINED(ABC)
    #include DEFINED(X)
    #include DEFINED(X, Y)

        strictlyEquals(new PatternSet(includes: ['i']), new PatternSet(includes: ['i']))
        strictlyEquals(new PatternSet(excludes: ['e']), new PatternSet(excludes: ['e']))
        strictlyEquals(new PatternSet(includes: ['i'], excludes: ['e']), new PatternSet(includes: ['i'], excludes: ['e']))

        new PatternSet() != new PatternSet(caseSensitive: false)
        new PatternSet() != new PatternSet(includes: ['i'])

                // Source file has not changed and no include files
                return true;
            }

            // Check each unique edge in the include file graph
            Map<HashCode, File> includes = new HashMap<HashCode, File>(previousState.getEdges().size());
            Set<File> headers = new HashSet<File>();
            includes.put(fileHash, sourceFile);

Using this mechanism, the included build may also contribute a settings plugin that can be applied in the settings file itself:

====
include::sample[dir="snippets/developingPlugins/testingPlugins/kotlin/include-plugin-build",files="settings.gradle.kts[tags=include-build]"]

     * @see org.gradle.api.tasks.util.PatternFilterable Pattern Format
     */
    @Override
    CopySpec include(String... includes);

    /**
     * {@inheritDoc}
     *
     * @see org.gradle.api.tasks.util.PatternFilterable Pattern Format
     */
    @Override
    CopySpec include(Iterable<String> includes);

    /**
     * {@inheritDoc}
     *

ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/llvm-3.6/bin/../lib/clang/3.6.0/include
 /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
"""

     * <pre class='autoTestedSettings'>
     *   // include two projects, 'foo' and 'foo:bar'
     *   // directories are inferred by replacing ':' with '/'
     *   include 'foo:bar'
     *
     *   // include one project whose project dir does not match the logical project path
     *   include 'baz'
     *   project(':baz').projectDir = file('foo/baz')
     *

In this example, the root settings file will look as follows:

.settings.gradle.kts
[source,kotlin]
----
include("mobile-app", "web-app", "api", "lib", "documentation")
----

NOTE: The order in which the subprojects (modules) are included does not matter.

The `buildSrc` directory is automatically recognized by Gradle.