data List:
	| link(first,rest)
	| empty()
end
let mt = empty()

data Option:
    | some(v)
    | none()
end

def singles(lst, to_replace):
    match to_replace:
        | some(v) => 
            match lst:
                | link(f,r) => 
                    if f == v:
                        singles(r, to_replace)
                    else:
                        link(f, singles(r, some(f)))
                    end
                | empty() => empty()
            end
        | none() =>
            match lst:
                | link(f,r) => link(f, singles(r, some(f)))
                | empty() => empty()
            end
    end
end

let one_through_four = link(1,link(2,link(3,link(4,mt))))
let with_dups1 = link(1,link(1,link(2,link(2,link(3,link(4,mt))))))
let with_dups2 = link(1,link(2,link(3,link(3,link(3,link(4,mt))))))

singles(mt, none()) == mt

singles(one_through_four, none()) == one_through_four

singles(with_dups1, none()) == one_through_four

singles(with_dups2, none()) == one_through_four