diff --git a/doc.go b/doc.go
index 694a978..2d67083 100644
--- a/doc.go
+++ b/doc.go
@@ -3,8 +3,7 @@
 // See the file LICENSE in this distribution for more details.
 
 /*
-Package github.com/sttk/cliargs is a library to parse command line
-arguments.
+Package github.com/sttk/cliargs is a library to parse command line arguments.
 
 # Parse without configurations
 
@@ -15,8 +14,8 @@ command arguments.
 
 Command line arguments starting with - or -- are options, and others are
 command arguments.
-If you want to specify a value to an option, follows "=" after the option like
-foo=123.
+If you want to specify a value to an option, follows "=" and the value after
+the option, like foo=123.
 
 All command line arguments after -- are command arguments, even they starts
 with - or --.
@@ -211,8 +210,10 @@ The following help text is generated from the above optCfgs (without Help#Print
 
 # Parse command line arguments including sub commands
 
-This library provides the function FindFirstArg which returns an index, an argument, an existent flag.
-This function can be used to parse command line arguments including sub commands, as follows:
+This library provides the function FindFirstArg which returns an index, an
+argument, an existent flag.
+This function can be used to parse command line arguments including sub
+commands, as follows:
 
 	i, arg, exists := cliargs.FindFirstArg(osArgs)
 	if !exists { return }
diff --git a/parse-for.go b/parse-for.go
index 0eb68d6..885c1d1 100644
--- a/parse-for.go
+++ b/parse-for.go
@@ -17,6 +17,7 @@ import (
 // command line arguments, is not a pointer.
 type OptionStoreIsNotChangeable struct{}
 
+// Error is the method to retrieve the message of this error.
 func (e OptionStoreIsNotChangeable) Error() string {
 	return "OptionStoreIsNotChangeable{}"
 }
@@ -31,12 +32,14 @@ type FailToParseInt struct {
 	cause   error
 }
 
+// Error is the method to retrieve the message of this error.
 func (e FailToParseInt) Error() string {
 	return fmt.Sprintf("FailToParseInt{"+
 		"Option:%s,Field:%s,Input:%s,BitSize:%d,cause:%s}",
 		e.Option, e.Field, e.Input, e.BitSize, e.cause.Error())
 }
 
+// Unwrap is the method to retrieve an error that caused this error.
 func (e FailToParseInt) Unwrap() error {
 	return e.cause
 }
@@ -51,12 +54,14 @@ type FailToParseUint struct {
 	cause   error
 }
 
+// Error is the method to retrieve the message of this error.
 func (e FailToParseUint) Error() string {
 	return fmt.Sprintf("FailToParseUint{"+
 		"Option:%s,Field:%s,Input:%s,BitSize:%d,cause:%s}",
 		e.Option, e.Field, e.Input, e.BitSize, e.cause.Error())
 }
 
+// Unwrap is the method to retrieve an error that caused this error.
 func (e FailToParseUint) Unwrap() error {
 	return e.cause
 }
@@ -71,12 +76,14 @@ type FailToParseFloat struct {
 	cause   error
 }
 
+// Error is the method to retrieve the message of this error.
 func (e FailToParseFloat) Error() string {
 	return fmt.Sprintf("FailToParseFloat{"+
 		"Option:%s,Field:%s,Input:%s,BitSize:%d,cause:%s}",
 		e.Option, e.Field, e.Input, e.BitSize, e.cause.Error())
 }
 
+// Unwrap is the method to retrieve an error that caused this error.
 func (e FailToParseFloat) Unwrap() error {
 	return e.cause
 }
@@ -90,6 +97,7 @@ type IllegalOptionType struct {
 	Type   reflect.Type
 }
 
+// Error is the method to retrieve the message of this error.
 func (e IllegalOptionType) Error() string {
 	return fmt.Sprintf("IllegalOptionType{"+
 		"Option:%s,Field:%s,Type:%s}",
@@ -133,10 +141,10 @@ func (e IllegalOptionType) Error() string {
 // before the open square bracket, like :[elem1:elem2:elem3].
 // It's useful when some array elements include commas.
 //
-// NOTE: A default value of a string array option in a struct tag is [], like
-// `opt:"name=[]"`, it doesn't represent an array which contains only an empty
-// string but an empty array.
-// If you want to specify an array which contains only an empty string, write
+// NOTE: A default value of an empty string array option in a struct tag is [],
+// like `opt:"name=[]"`, it doesn't represent an array which contains only one
+// empty string but an empty array.
+// If you want to specify an array which contains only one empty string, write
 // nothing after "=" mark, like `opt:"name="`.
 func ParseFor(osArgs []string, options any) (Cmd, []OptCfg, error) {
 	optCfgs, err := MakeOptCfgsFor(options)
diff --git a/parse-with.go b/parse-with.go
index b13e728..2f58579 100644
--- a/parse-with.go
+++ b/parse-with.go
@@ -100,7 +100,7 @@ func (e OptionIsNotArray) GetOpt() string {
 
 const anyOption = "*"
 
-// OptCfg is the structure that represents an option configuration.
+// OptCfg is the struct that represents an option configuration.
 // An option configuration consists of fields: Name, Aliases, HasArg,
 // IsArray, Defaults, OnParsed, Desc, and ArgHelp.
 //