Note that there are 3 different implementations for removal of duplicates.

```
object Example {
case class Book(title: String, authors: String*)
val books: List[Book] = List(
Book(
"Structure and Interpretation of Computer Programs",
"Abelson, Harold", "Sussman, Gerald J."
),
Book(
"Principles of Compiler Design",
"Aho, Alfred", "Ullman, Jeffrey"
),
Book(
"Programming in Modula-2",
"Wirth, Niklaus"
),
Book(
"Elements of ML Programming",
"Ullman, Jeffrey"
),
Book(
"The Java Language Specification", "Gosling, James",
"Joy, Bill", "Steele, Guy", "Bracha, Gilad"
)
)
}
object Test {
import Example._
for {
b <- books
a <- b.authors
if a startsWith("Gosling")
} yield b.title
for {
b <- books
if (b.title indexOf "Program" ) >= 0
} yield b.title
for {
b1 <- books
b2 <- books
if b1 != b2
a1 <- b1.authors
a2 <- b2.authors
if a1 == a2
} yield a1
def removeDuplicated[A](xs: List[A]): List[A] = {
if(xs.isEmpty) xs
else {
val y = removeDuplicated(xs.tail)
if(y.contains(xs.head)) y
else xs.head :: y
}
}
def removeDuplicated1[A](xs: List[A]): List[A] = {
if(xs.isEmpty) xs
else {
xs.head :: removeDuplicated1(
xs.tail.filter(x => x != xs.head)
)
}
}
def removeDuplicated2[A](xs: List[A]): List[A] = {
if(xs.isEmpty) xs
else {
xs.head :: removeDuplicated2(
for {
x <- xs.tail
if x != xs.head
} yield x
)
}
}
removeDuplicated(List(1, 1, 2, 2, 3))
removeDuplicated1(List(1, 1, 2, 2, 3))
removeDuplicated2(List(1, 1, 2, 2, 3))
val profilicAuthors = for {
b1 <- books
b2 <- books
if b1 != b2
a1 <- b1.authors
a2 <- b2.authors
if a1 == a2
} yield a1
removeDuplicated(profilicAuthors)
removeDuplicated1(profilicAuthors)
removeDuplicated2(profilicAuthors)
}
```

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