List Insert function error

[Rholar]

This program takes in two ordered lists and then combines them into one ordered list, then returns that new list.
I am having trouble with the insert function. I get this error:
error: no matching function for call to ‘std::list<int>::insert(std::list<int>::const_iterator&, const int&)’
copy.insert(iter1, *iter2);

I looked at the function parameters for insert and it is:
single element (1) iterator insert (iterator position, const value_type& val);

I believe i have the "const value_type& val" right because copy.push_back(*iter2) works and it has the same parameter code:
void push_back (const value_type& val);

So it is either copy that is messing it up or the "const_iterator&" parameter i have, i believe.
I know there are other ways of doing this program with just push_back and i know how to do it like that but i want to know why this solution doesn't work or at least why mine won't work.

Thank you.

#include <list>
#include <algorithm>
#include <iostream>
#include <cassert>

using namespace std;

template <class T>
list<T> ordered_union(const list<T> & a, const list<T> & b)
{
    list<T> copy = a;  //makes a copy of list a

    typename list<T>::const_iterator iter1 = a.begin();
    typename list<T>::const_iterator iter2 = b.begin();

    for(;iter1 != a.end();) {
        for(; iter2 != b.end();)
            if(*iter2 <= *iter1) {  // if iter2 value is less than iter1 it gets inserted at iter1's position in copy
                copy.insert(iter1, *iter2);
                iter2++;
            }
            else {
                iter1++;
                break;
            }
    }
    for(;iter2 != b.end();iter2++)  //if iter2 is longer than iter1
        copy.push_back(*iter2);
    return copy;

}

///other functions and main past here

[Peter87]

Before C++11 you couldn't pass const_iterator to the insert function. If you're not using C++11 (or later) you will have to use non-const iterators instead.