override fun visitFileTree(root: File, patterns: PatternSet, fileTree: FileTreeInternal) {
            roots.add(DirectoryTreeSpec(root, patterns))
        }

        override fun visitFileTreeBackedByFile(file: File, fileTree: FileTreeInternal, sourceTree: FileSystemMirroringFileTree) {
            roots.add(toSpec(sourceTree))
        }
    }

    private

abstract class AbstractVisitor : FileCollectionStructureVisitor {
    override fun visitFileTree(root: File, patterns: PatternSet, fileTree: FileTreeInternal) =
        unsupportedFileTree(fileTree)

    override fun visitFileTreeBackedByFile(file: File, fileTree: FileTreeInternal, sourceTree: FileSystemMirroringFileTree) =
        unsupportedFileTree(fileTree)

    private
    fun unsupportedFileTree(fileTree: FileTreeInternal): Nothing =

        override fun visitFileTree(root: File, patterns: PatternSet, fileTree: FileTreeInternal) {
            elements.add(directoryFileTreeFactory.create(root, patterns))
        }

        override fun visitFileTreeBackedByFile(file: File, fileTree: FileTreeInternal, sourceTree: FileSystemMirroringFileTree) {
            elements.add(file)
        }

        fun simplify(): FileCollectionInternal = fileCollectionFactory.resolving(elements)