diff --git a/atom.go b/atom.go
index 55d0532..70d90ed 100644
--- a/atom.go
+++ b/atom.go
@@ -1,6 +1,7 @@
 package atom
 
 import (
+	"encoding/xml"
 	"fmt"
 	"html"
 	"mime"
@@ -11,6 +12,29 @@ import (
 	"golang.org/x/text/language"
 )
 
+type Countable interface {
+	*xml.Attr | *Person | *Category | *Link | *ExtensionElement | *Entry
+}
+
+func addToSlice[C Countable](slice []C, countable C) {
+	if slice == nil {
+		slice = make([]C, 1)
+		slice[0] = countable
+	} else {
+		slice = append(slice, countable)
+	}
+}
+
+func deleteFromSlice[C Countable](slice []C, id int) error {
+	length := len(slice)
+	if id > length {
+		return fmt.Errorf("id %v out of range %v", id, length)
+	}
+
+	slice = append(slice[:id], slice[id+1:]...)
+	return nil
+}
+
 // isValidIRI checks whether an IRI is valid or not. It returns a bool.
 // https://www.w3.org/2011/04/XMLSchema/TypeLibrary-IRI-RFC3987.xsd
 func isValidIRI(iri string) bool {
diff --git a/commonAttributes.go b/commonAttributes.go
index 875eea7..e8d5d2b 100644
--- a/commonAttributes.go
+++ b/commonAttributes.go
@@ -19,23 +19,15 @@ func newCommonAttributes() *CommonAttributes {
 
 // AddAttribute adds the attribute to the CommonAttributes.
 func (c *CommonAttributes) AddAttribute(name, value string) {
-	if c.UndefinedAttributes == nil {
-		c.UndefinedAttributes = make([]*xml.Attr, 1)
-		c.UndefinedAttributes[0] = &xml.Attr{Name: xml.Name{Local: name}, Value: value}
-	} else {
-		c.UndefinedAttributes = append(c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
-	}
+	addToSlice(c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
 }
 
 // DeleteAttribute deletes the attribute from the CommonAttributes. It return an
 // error.
 func (c *CommonAttributes) DeleteAttribute(id int) error {
-	length := len(c.UndefinedAttributes)
-	if id > length {
-		return fmt.Errorf("error deleting undefined attribute from common attributes %v: id %v out of range %v", c, id, length)
+	if err := deleteFromSlice(c.UndefinedAttributes, id); err != nil {
+		return fmt.Errorf("error deleting undefined attribute %v from common attributes %v: %v", id, c, err)
 	}
-
-	c.UndefinedAttributes = append(c.UndefinedAttributes[:id], c.UndefinedAttributes[id+1:]...)
 	return nil
 }
 
diff --git a/entry.go b/entry.go
index 6b74f5f..b2fe95e 100644
--- a/entry.go
+++ b/entry.go
@@ -68,120 +68,80 @@ func NewEntry(title string) *Entry {
 
 // AddAuthor adds the Person as an author to the Entry.
 func (e *Entry) AddAuthor(p *Person) {
-	if e.Authors == nil {
-		e.Authors = make([]*Person, 1)
-		e.Authors[0] = p
-	} else {
-		e.Authors = append(e.Authors, p)
-	}
-
+	addToSlice(e.Authors, p)
 	e.Updated = NewDate(time.Now())
 }
 
 // DeleteAuthor deletes the Person from the Entry. It return an error.
 func (e *Entry) DeleteAuthor(id int) error {
-	length := len(e.Authors)
-	if id > length {
-		return fmt.Errorf("error deleting author from entry %v: id %v out of range %v", e.ID.URI, id, length)
+	if err := deleteFromSlice(e.Authors, id); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", id, e.ID.URI, err)
 	}
 
-	e.Authors = append(e.Authors[:id], e.Authors[id+1:]...)
 	e.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddCategory adds the Category to the Entry.
 func (e *Entry) AddCategory(c *Category) {
-	if e.Categories == nil {
-		e.Categories = make([]*Category, 1)
-		e.Categories[0] = c
-	} else {
-		e.Categories = append(e.Categories, c)
-	}
-
+	addToSlice(e.Categories, c)
 	e.Updated = NewDate(time.Now())
 }
 
 // DeleteCategory deletes the Category from the Entry. It return an error.
 func (e *Entry) DeleteCategory(id int) error {
-	length := len(e.Categories)
-	if id > length {
-		return fmt.Errorf("error deleting category from entry %v: id %v out of range %v", e.ID.URI, id, length)
+	if err := deleteFromSlice(e.Categories, id); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", id, e.ID.URI, err)
 	}
 
-	e.Categories = append(e.Categories[:id], e.Categories[id+1:]...)
 	e.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddContributor adds the Person as a contributor to the Entry.
 func (e *Entry) AddContributor(c *Person) {
-	if e.Contributors == nil {
-		e.Contributors = make([]*Person, 1)
-		e.Contributors[0] = c
-	} else {
-		e.Contributors = append(e.Contributors, c)
-	}
-
+	addToSlice(e.Contributors, c)
 	e.Updated = NewDate(time.Now())
 }
 
 // DeleteContributor deletes the Person from the Entry. It return an error.
 func (e *Entry) DeleteContributor(id int) error {
-	length := len(e.Contributors)
-	if id > length {
-		return fmt.Errorf("error deleting contributor from entry %v: id %v out of range %v", e.ID.URI, id, length)
+	if err := deleteFromSlice(e.Contributors, id); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", id, e.ID.URI, err)
 	}
 
-	e.Contributors = append(e.Contributors[:id], e.Contributors[id+1:]...)
 	e.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddLink adds the Link to the Entry.
 func (e *Entry) AddLink(l *Link) {
-	if e.Links == nil {
-		e.Links = make([]*Link, 1)
-		e.Links[0] = l
-	} else {
-		e.Links = append(e.Links, l)
-	}
-
+	addToSlice(e.Links, l)
 	e.Updated = NewDate(time.Now())
 }
 
 // DeleteLink deletes the Link from the Entry. It return an error.
 func (e *Entry) DeleteLink(id int) error {
-	length := len(e.Links)
-	if id > length {
-		return fmt.Errorf("error deleting link from entry %v: id %v out of range %v", e.ID.URI, id, length)
+	if err := deleteFromSlice(e.Links, id); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", id, e.ID.URI, err)
 	}
 
-	e.Links = append(e.Links[:id], e.Links[id+1:]...)
 	e.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddExtension adds the ExtensionElement to the Entry.
 func (e *Entry) AddExtension(x *ExtensionElement) {
-	if e.Extensions == nil {
-		e.Extensions = make([]*ExtensionElement, 1)
-		e.Extensions[0] = x
-	} else {
-		e.Extensions = append(e.Extensions, x)
-	}
-
+	addToSlice(e.Extensions, x)
 	e.Updated = NewDate(time.Now())
 }
 
 // DeleteExtension deletes the Extension from the Entry. It return an error.
 func (e *Entry) DeleteExtension(id int) error {
-	length := len(e.Extensions)
-	if id > length {
-		return fmt.Errorf("error deleting extension from entry %v: id %v out of range %v", e.ID.URI, id, length)
+	if err := deleteFromSlice(e.Extensions, id); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", id, e.ID.URI, err)
 	}
 
-	e.Extensions = append(e.Extensions[:id], e.Extensions[id+1:]...)
 	e.Updated = NewDate(time.Now())
 	return nil
 }
diff --git a/feed.go b/feed.go
index 1ba637d..b496192 100644
--- a/feed.go
+++ b/feed.go
@@ -37,144 +37,96 @@ func NewFeed(title string) *Feed {
 
 // AddAuthor adds the Person as an author to the Feed.
 func (f *Feed) AddAuthor(p *Person) {
-	if f.Authors == nil {
-		f.Authors = make([]*Person, 1)
-		f.Authors[0] = p
-	} else {
-		f.Authors = append(f.Authors, p)
-	}
-
+	addToSlice(f.Authors, p)
 	f.Updated = NewDate(time.Now())
 }
 
 // DeleteAuthor deletes the Person from the Feed. It return an error.
 func (f *Feed) DeleteAuthor(id int) error {
-	length := len(f.Authors)
-	if id > length {
-		return fmt.Errorf("error deleting author from feed %v: id %v out of range %v", f.ID.URI, id, length)
+	if err := deleteFromSlice(f.Authors, id); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", id, f.ID.URI, err)
 	}
 
-	f.Authors = append(f.Authors[:id], f.Authors[id+1:]...)
 	f.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddCategory adds the Category to the Feed.
 func (f *Feed) AddCategory(c *Category) {
-	if f.Categories == nil {
-		f.Categories = make([]*Category, 1)
-		f.Categories[0] = c
-	} else {
-		f.Categories = append(f.Categories, c)
-	}
-
+	addToSlice(f.Categories, c)
 	f.Updated = NewDate(time.Now())
 }
 
 // DeleteCategory deletes the Category from the Feed. It return an error.
 func (f *Feed) DeleteCategory(id int) error {
-	length := len(f.Categories)
-	if id > length {
-		return fmt.Errorf("error deleting category from feed %v: id %v out of range %v", f.ID.URI, id, length)
+	if err := deleteFromSlice(f.Categories, id); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", id, f.ID.URI, err)
 	}
 
-	f.Categories = append(f.Categories[:id], f.Categories[id+1:]...)
 	f.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddContributor adds the Person as a contributor to the Feed.
 func (f *Feed) AddContributor(c *Person) {
-	if f.Contributors == nil {
-		f.Contributors = make([]*Person, 1)
-		f.Contributors[0] = c
-	} else {
-		f.Contributors = append(f.Contributors, c)
-	}
-
+	addToSlice(f.Contributors, c)
 	f.Updated = NewDate(time.Now())
 }
 
 // DeleteContributor deletes the Person from the Feed. It return an error.
 func (f *Feed) DeleteContributor(id int) error {
-	length := len(f.Contributors)
-	if id > length {
-		return fmt.Errorf("error deleting contributor from feed %v: id %v out of range %v", f.ID.URI, id, length)
+	if err := deleteFromSlice(f.Contributors, id); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", id, f.ID.URI, err)
 	}
 
-	f.Contributors = append(f.Contributors[:id], f.Contributors[id+1:]...)
 	f.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddLink adds the Link to the Feed. There should be one Link with Rel "self".
 func (f *Feed) AddLink(l *Link) {
-	if f.Links == nil {
-		f.Links = make([]*Link, 1)
-		f.Links[0] = l
-	} else {
-		f.Links = append(f.Links, l)
-	}
-
+	addToSlice(f.Links, l)
 	f.Updated = NewDate(time.Now())
 }
 
 // DeleteLink deletes the Link from the Feed. It return an error.
 func (f *Feed) DeleteLink(id int) error {
-	length := len(f.Links)
-	if id > length {
-		return fmt.Errorf("error deleting link from feed %v: id %v out of range %v", f.ID.URI, id, length)
+	if err := deleteFromSlice(f.Links, id); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", id, f.ID.URI, err)
 	}
 
-	f.Links = append(f.Links[:id], f.Links[id+1:]...)
 	f.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddExtension adds the Extension to the Feed.
 func (f *Feed) AddExtension(e *ExtensionElement) {
-	if f.Extensions == nil {
-		f.Extensions = make([]*ExtensionElement, 1)
-		f.Extensions[0] = e
-	} else {
-		f.Extensions = append(f.Extensions, e)
-	}
-
+	addToSlice(f.Extensions, e)
 	f.Updated = NewDate(time.Now())
 }
 
 // DeleteExtension deletes the Extension from the Feed. It return an error.
 func (f *Feed) DeleteExtension(id int) error {
-	length := len(f.Extensions)
-	if id > length {
-		return fmt.Errorf("error deleting extension from feed %v: id %v out of range %v", f.ID.URI, id, length)
+	if err := deleteFromSlice(f.Extensions, id); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", id, f.ID.URI, err)
 	}
 
-	f.Extensions = append(f.Extensions[:id], f.Extensions[id+1:]...)
 	f.Updated = NewDate(time.Now())
 	return nil
 }
 
 // AddEntry adds the Entry to the Feed.
 func (f *Feed) AddEntry(e *Entry) {
-	if f.Entries == nil {
-		f.Entries = make([]*Entry, 1)
-		f.Entries[0] = e
-	} else {
-		f.Entries = append(f.Entries, e)
-	}
-
+	addToSlice(f.Entries, e)
 	f.Updated = NewDate(time.Now())
 }
 
 // DeleteEntry deletes the Entry from the Feed. It return an error.
 func (f *Feed) DeleteEntry(id int) error {
-	length := len(f.Entries)
-	if id > length {
-		return fmt.Errorf("error deleting entry from feed %v: id %v out of range %v", f.ID.URI, id, length)
+	if err := deleteFromSlice(f.Entries, id); err != nil {
+		return fmt.Errorf("error deleting entry %v from entry %v: %v", id, f.ID.URI, err)
 	}
 
-	f.Entries = append(f.Entries[:id], f.Entries[id+1:]...)
 	f.Updated = NewDate(time.Now())
 	return nil
 }
diff --git a/person.go b/person.go
index 2864681..5505698 100644
--- a/person.go
+++ b/person.go
@@ -23,22 +23,14 @@ func NewPerson(name string) *Person {
 
 // AddExtension adds the Extension to the Person.
 func (p *Person) AddExtension(e *ExtensionElement) {
-	if p.Extensions == nil {
-		p.Extensions = make([]*ExtensionElement, 1)
-		p.Extensions[0] = e
-	} else {
-		p.Extensions = append(p.Extensions, e)
-	}
+	addToSlice(p.Extensions, e)
 }
 
 // DeleteExtension deletes the Extension from the Person. It return an error.
 func (p *Person) DeleteExtension(id int) error {
-	length := len(p.Extensions)
-	if id > length {
-		return fmt.Errorf("error deleting extension from person %v: id %v out of range %v", p, id, length)
+	if err := deleteFromSlice(p.Extensions, id); err != nil {
+		return fmt.Errorf("error deleting extension %v from person %v: %v", id, p, err)
 	}
-
-	p.Extensions = append(p.Extensions[:id], p.Extensions[id+1:]...)
 	return nil
 }
 
diff --git a/source.go b/source.go
index 3af7601..4f175b9 100644
--- a/source.go
+++ b/source.go
@@ -30,106 +30,66 @@ func NewSource() *Source {
 
 // AddAuthor adds the Person as an author to the Source.
 func (s *Source) AddAuthor(p *Person) {
-	if s.Authors == nil {
-		s.Authors = make([]*Person, 1)
-		s.Authors[0] = p
-	} else {
-		s.Authors = append(s.Authors, p)
-	}
+	addToSlice(s.Authors, p)
 }
 
 // DeleteAuthor deletes the Person from the Source. It return an error.
 func (s *Source) DeleteAuthor(id int) error {
-	length := len(s.Authors)
-	if id > length {
-		return fmt.Errorf("error deleting author from source %v: id %v out of range %v", s, id, length)
+	if err := deleteFromSlice(s.Authors, id); err != nil {
+		return fmt.Errorf("error deleting author %v from source %v: %v", id, s, err)
 	}
-
-	s.Authors = append(s.Authors[:id], s.Authors[id+1:]...)
 	return nil
 }
 
 // AddCategory adds the Category to the Source.
 func (s *Source) AddCategory(c *Category) {
-	if s.Categories == nil {
-		s.Categories = make([]*Category, 1)
-		s.Categories[0] = c
-	} else {
-		s.Categories = append(s.Categories, c)
-	}
+	addToSlice(s.Categories, c)
 }
 
 // DeleteCategory deletes the Category from the Source. It return an error.
 func (s *Source) DeleteCategory(id int) error {
-	length := len(s.Categories)
-	if id > length {
-		return fmt.Errorf("error deleting category from source %v: id %v out of range %v", s, id, length)
+	if err := deleteFromSlice(s.Categories, id); err != nil {
+		return fmt.Errorf("error deleting category %v from source %v: %v", id, s, err)
 	}
-
-	s.Categories = append(s.Categories[:id], s.Categories[id+1:]...)
 	return nil
 }
 
 // AddContributor adds the Person as a contributor to the Source.
 func (s *Source) AddContributor(c *Person) {
-	if s.Contributors == nil {
-		s.Contributors = make([]*Person, 1)
-		s.Contributors[0] = c
-	} else {
-		s.Contributors = append(s.Contributors, c)
-	}
+	addToSlice(s.Contributors, c)
 }
 
 // DeleteContributor deletes the Person from the Source. It return an error.
 func (s *Source) DeleteContributor(id int) error {
-	length := len(s.Contributors)
-	if id > length {
-		return fmt.Errorf("error deleting contributor from source %v: id %v out of range %v", s, id, length)
+	if err := deleteFromSlice(s.Contributors, id); err != nil {
+		return fmt.Errorf("error deleting contributor %v from source %v: %v", id, s, err)
 	}
-
-	s.Contributors = append(s.Contributors[:id], s.Contributors[id+1:]...)
 	return nil
 }
 
 // AddLink adds the Link to the Source.
 func (s *Source) AddLink(l *Link) {
-	if s.Links == nil {
-		s.Links = make([]*Link, 1)
-		s.Links[0] = l
-	} else {
-		s.Links = append(s.Links, l)
-	}
+	addToSlice(s.Links, l)
 }
 
 // DeleteLink deletes the Link from the Source. It return an error.
 func (s *Source) DeleteLink(id int) error {
-	length := len(s.Links)
-	if id > length {
-		return fmt.Errorf("error deleting link from source %v: id %v out of range %v", s, id, length)
+	if err := deleteFromSlice(s.Links, id); err != nil {
+		return fmt.Errorf("error deleting link %v from source %v: %v", id, s, err)
 	}
-
-	s.Links = append(s.Links[:id], s.Links[id+1:]...)
 	return nil
 }
 
 // AddExtension adds the ExtensionElement to the Source.
 func (s *Source) AddExtension(e *ExtensionElement) {
-	if s.Extensions == nil {
-		s.Extensions = make([]*ExtensionElement, 1)
-		s.Extensions[0] = e
-	} else {
-		s.Extensions = append(s.Extensions, e)
-	}
+	addToSlice(s.Extensions, e)
 }
 
 // DeleteExtension deletes the Extension from the Source. It return an error.
 func (s *Source) DeleteExtension(id int) error {
-	length := len(s.Extensions)
-	if id > length {
-		return fmt.Errorf("error deleting extension from source %v: id %v out of range %v", s, id, length)
+	if err := deleteFromSlice(s.Extensions, id); err != nil {
+		return fmt.Errorf("error deleting extension %v from source %v: %v", id, s, err)
 	}
-
-	s.Extensions = append(s.Extensions[:id], s.Extensions[id+1:]...)
 	return nil
 }