diff --git a/io.go b/io.go
index 3c3634f..5c5bcc8 100644
--- a/io.go
+++ b/io.go
@@ -385,6 +385,9 @@ func (medium *MemoryMedium) WriteStream(path string) (goio.WriteCloser, error) {
 	return medium.Create(path)
 }
 
+// Example: medium := io.NewMemoryMedium()
+// Example: _ = medium.Write("config/app.yaml", "port: 8080")
+// Example: file, _ := medium.Open("config/app.yaml")
 type MemoryFile struct {
 	name    string
 	content []byte
@@ -408,6 +411,8 @@ func (file *MemoryFile) Close() error {
 	return nil
 }
 
+// Example: medium := io.NewMemoryMedium()
+// Example: writer, _ := medium.Create("logs/app.log")
 type MemoryWriteCloser struct {
 	medium *MemoryMedium
 	path   string
diff --git a/s3/s3.go b/s3/s3.go
index 5baf152..7dc6bb5 100644
--- a/s3/s3.go
+++ b/s3/s3.go
@@ -90,10 +90,10 @@ func normalisePrefix(prefix string) string {
 // Example: _ = medium.Write("reports/daily.txt", "done")
 func New(options Options) (*Medium, error) {
 	if options.Bucket == "" {
-		return nil, core.E("s3.New", "bucket name is required", nil)
+		return nil, core.E("s3.New", "bucket name is required", fs.ErrInvalid)
 	}
 	if options.Client == nil {
-		return nil, core.E("s3.New", "client is required", nil)
+		return nil, core.E("s3.New", "client is required", fs.ErrInvalid)
 	}
 	medium := &Medium{
 		client: options.Client,
diff --git a/sigil/crypto_sigil.go b/sigil/crypto_sigil.go
index 306f702..f57e351 100644
--- a/sigil/crypto_sigil.go
+++ b/sigil/crypto_sigil.go
@@ -23,12 +23,14 @@ var (
 	NoKeyConfiguredError = core.E("sigil.NoKeyConfiguredError", "no encryption key configured", nil)
 )
 
+// Example: obfuscator := &sigil.XORObfuscator{}
 type PreObfuscator interface {
 	Obfuscate(data []byte, entropy []byte) []byte
 
 	Deobfuscate(data []byte, entropy []byte) []byte
 }
 
+// Example: obfuscator := &sigil.XORObfuscator{}
 type XORObfuscator struct{}
 
 func (obfuscator *XORObfuscator) Obfuscate(data []byte, entropy []byte) []byte {
@@ -76,6 +78,7 @@ func (obfuscator *XORObfuscator) deriveKeyStream(entropy []byte, length int) []b
 	return stream
 }
 
+// Example: obfuscator := &sigil.ShuffleMaskObfuscator{}
 type ShuffleMaskObfuscator struct{}
 
 func (obfuscator *ShuffleMaskObfuscator) Obfuscate(data []byte, entropy []byte) []byte {
diff --git a/sigil/sigils.go b/sigil/sigils.go
index 1dd9983..c13c159 100644
--- a/sigil/sigils.go
+++ b/sigil/sigils.go
@@ -11,6 +11,7 @@ import (
 	"encoding/base64"
 	"encoding/hex"
 	goio "io"
+	"io/fs"
 
 	core "dappco.re/go/core"
 	"golang.org/x/crypto/blake2b"
@@ -20,6 +21,7 @@ import (
 	"golang.org/x/crypto/sha3"
 )
 
+// Example: reverseSigil, _ := sigil.NewSigil("reverse")
 type ReverseSigil struct{}
 
 func (sigil *ReverseSigil) In(data []byte) ([]byte, error) {
@@ -37,6 +39,7 @@ func (sigil *ReverseSigil) Out(data []byte) ([]byte, error) {
 	return sigil.In(data)
 }
 
+// Example: hexSigil, _ := sigil.NewSigil("hex")
 type HexSigil struct{}
 
 func (sigil *HexSigil) In(data []byte) ([]byte, error) {
@@ -57,6 +60,7 @@ func (sigil *HexSigil) Out(data []byte) ([]byte, error) {
 	return dst, err
 }
 
+// Example: base64Sigil, _ := sigil.NewSigil("base64")
 type Base64Sigil struct{}
 
 func (sigil *Base64Sigil) In(data []byte) ([]byte, error) {
@@ -77,6 +81,7 @@ func (sigil *Base64Sigil) Out(data []byte) ([]byte, error) {
 	return dst[:n], err
 }
 
+// Example: gzipSigil, _ := sigil.NewSigil("gzip")
 type GzipSigil struct {
 	outputWriter goio.Writer
 }
@@ -116,6 +121,7 @@ func (sigil *GzipSigil) Out(data []byte) ([]byte, error) {
 	return out, nil
 }
 
+// Example: jsonSigil := &sigil.JSONSigil{Indent: true}
 type JSONSigil struct{ Indent bool }
 
 func (sigil *JSONSigil) In(data []byte) ([]byte, error) {
@@ -129,7 +135,7 @@ func (sigil *JSONSigil) In(data []byte) ([]byte, error) {
 		if err, ok := result.Value.(error); ok {
 			return nil, core.E("sigil.JSONSigil.In", "decode json", err)
 		}
-		return nil, core.E("sigil.JSONSigil.In", "decode json", nil)
+		return nil, core.E("sigil.JSONSigil.In", "decode json", fs.ErrInvalid)
 	}
 
 	compact := core.JSONMarshalString(decoded)
@@ -143,6 +149,7 @@ func (sigil *JSONSigil) Out(data []byte) ([]byte, error) {
 	return data, nil
 }
 
+// Example: hashSigil := sigil.NewHashSigil(crypto.SHA256)
 type HashSigil struct {
 	Hash crypto.Hash
 }
@@ -193,7 +200,7 @@ func (sigil *HashSigil) In(data []byte) ([]byte, error) {
 	case crypto.BLAKE2b_512:
 		hasher, _ = blake2b.New512(nil)
 	default:
-		return nil, core.E("sigil.HashSigil.In", "hash algorithm not available", nil)
+		return nil, core.E("sigil.HashSigil.In", "hash algorithm not available", fs.ErrInvalid)
 	}
 
 	hasher.Write(data)
@@ -258,7 +265,7 @@ func NewSigil(name string) (Sigil, error) {
 	case "blake2b-512":
 		return NewHashSigil(crypto.BLAKE2b_512), nil
 	default:
-		return nil, core.E("sigil.NewSigil", core.Concat("unknown sigil name: ", name), nil)
+		return nil, core.E("sigil.NewSigil", core.Concat("unknown sigil name: ", name), fs.ErrInvalid)
 	}
 }
 
diff --git a/sqlite/sqlite.go b/sqlite/sqlite.go
index b3d4475..162a98a 100644
--- a/sqlite/sqlite.go
+++ b/sqlite/sqlite.go
@@ -25,6 +25,7 @@ type Medium struct {
 
 var _ coreio.Medium = (*Medium)(nil)
 
+// Example: medium, _ := sqlite.New(sqlite.Options{Path: ":memory:", Table: "files"})
 type Options struct {
 	Path  string
 	Table string
@@ -41,7 +42,7 @@ func normaliseTableName(table string) string {
 // Example: _ = medium.Write("config/app.yaml", "port: 8080")
 func New(options Options) (*Medium, error) {
 	if options.Path == "" {
-		return nil, core.E("sqlite.New", "database path is required", nil)
+		return nil, core.E("sqlite.New", "database path is required", fs.ErrInvalid)
 	}
 
 	medium := &Medium{table: normaliseTableName(options.Table)}