diff --git a/internal/rosa/rosa.go b/internal/rosa/rosa.go
index e49bba5..7c0e3d3 100644
--- a/internal/rosa/rosa.go
+++ b/internal/rosa/rosa.go
@@ -392,3 +392,154 @@ cat /usr/src/` + name + `-patches/* | \
 		t.Load(Patch),
 	), nil, nil, script, paths...)
 }
+
+// helperInPlace is a special directory value for omitting the cd statement.
+const helperInPlace = "\x00"
+
+// Helper is a build system helper for [Toolchain.NewPackage].
+type Helper interface {
+	// name returns the value passed to the name argument of [Toolchain.New].
+	name(name, version string) string
+	// extra returns helper-specific dependencies.
+	extra(flag int) []PArtifact
+
+	// wantsChmod returns whether the source directory should be made writable.
+	wantsChmod() bool
+	// wantsWrite returns whether the source directory should be mounted writable.
+	wantsWrite() bool
+	// scriptEarly returns the helper-specific segment of cure script that goes
+	// before the cd statement.
+	scriptEarly() string
+	// createDir returns whether the path returned by wantsDir should be created.
+	createDir() bool
+	// wantsDir returns the directory to enter before script.
+	//
+	// The zero value implies source directory if [PackageAttr.ScriptEarly] is
+	// also empty. The special value helperInPlace omits the cd statement.
+	wantsDir() string
+	// script returns the helper-specific segment of cure script.
+	script(name string) string
+}
+
+const (
+	// sourceTarXZ denotes a source tarball to be decompressed using [XZ].
+	sourceTarXZ = 1 + iota
+)
+
+// PackageAttr holds build-system-agnostic attributes.
+type PackageAttr struct {
+	// Mount the source tree writable.
+	Writable bool
+	// Do not pass through [Toolchain.NewPatchedSource].
+	Chmod bool
+	// Unconditionally enter source directory early.
+	EnterSource bool
+
+	// Additional environment variables.
+	Env []string
+	// Runs before script emitted by [Helper]. Enters source if non-empty.
+	ScriptEarly string
+
+	// Passed to [Toolchain.NewPatchedSource].
+	Patches [][2]string
+	// Kind of source artifact.
+	SourceKind int
+
+	// Dependencies not provided by stage0.
+	NonStage0 []pkg.Artifact
+
+	// Passed through to [Toolchain.New], before source.
+	Paths []pkg.ExecPath
+	// Passed through to [Toolchain.New].
+	Flag int
+}
+
+// NewPackage constructs a [pkg.Artifact] via a build system helper.
+func (t Toolchain) NewPackage(
+	name, version string,
+	source pkg.Artifact,
+	attr *PackageAttr,
+	helper Helper,
+	extra ...PArtifact,
+) pkg.Artifact {
+	if attr == nil {
+		attr = new(PackageAttr)
+	}
+
+	if name == "" || version == "" {
+		panic("name must be non-empty")
+	}
+	if source == nil {
+		panic("source must be non-nil")
+	}
+	wantsChmod, wantsWrite := helper.wantsChmod(), helper.wantsWrite()
+	if attr.SourceKind > 0 &&
+		(attr.Writable || attr.Chmod || wantsChmod || wantsWrite || len(attr.Patches) > 0) {
+		panic("source processing requested on a non-unpacked kind")
+	}
+
+	dc := len(attr.NonStage0)
+	if !t.isStage0() {
+		dc += 1<<3 + len(extra)
+	}
+
+	extraRes := make([]pkg.Artifact, 0, dc)
+	extraRes = append(extraRes, attr.NonStage0...)
+	if !t.isStage0() {
+		for _, p := range helper.extra(attr.Flag) {
+			extraRes = append(extraRes, t.Load(p))
+		}
+		for _, p := range extra {
+			extraRes = append(extraRes, t.Load(p))
+		}
+	}
+
+	var scriptEarly string
+
+	var sourceSuffix string
+	switch attr.SourceKind {
+	case sourceTarXZ:
+		sourceSuffix = ".tar.xz"
+		scriptEarly += `
+tar -C /usr/src/ -xf '/usr/src/` + name + `.tar.xz'
+mv '/usr/src/` + name + `-` + version + `' '/usr/src/` + name + `'
+`
+		break
+	}
+
+	dir := helper.wantsDir()
+	helperScriptEarly := helper.scriptEarly()
+	if attr.EnterSource ||
+		dir == "" ||
+		attr.ScriptEarly != "" ||
+		helperScriptEarly != "" {
+		scriptEarly += `
+cd '/usr/src/` + name + `/'
+`
+	}
+	scriptEarly += attr.ScriptEarly + helperScriptEarly
+	if dir != "" && dir != helperInPlace {
+		if helper.createDir() {
+			scriptEarly += "\nmkdir -p " + dir
+		}
+		scriptEarly += "\ncd " + dir + "\n"
+	} else if !attr.EnterSource && attr.ScriptEarly == "" {
+		panic("cannot remain in root")
+	}
+
+	return t.New(
+		helper.name(name, version),
+		attr.Flag,
+		extraRes,
+		nil,
+		attr.Env,
+		scriptEarly+helper.script(name),
+		slices.Concat(attr.Paths, []pkg.ExecPath{
+			pkg.Path(AbsUsrSrc.Append(
+				name+sourceSuffix,
+			), attr.Writable || wantsWrite, t.NewPatchedSource(
+				name, version, source, !attr.Chmod && !wantsChmod, attr.Patches...,
+			)),
+		})...,
+	)
+}