<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def post_comment(request, next=None, using=None):
 """ Post a comment. HTTP POST is required. """

    # Fill out some initial data fields from an authenticated user, if present
    data = request.POST.copy()
    if request.user.is_authenticated:
        data["user"] = request.user
    else:
        return CommentPostBadRequest("You must be logged in to comment")

    # Look up the object we're trying to comment about
    ctype = data.get("content_type")
    object_pk = data.get("object_pk")
    if ctype is None or object_pk is None:
        return CommentPostBadRequest("Missing content_type or object_pk field.")
    try:
        model = models.get_model(*ctype.split(".", 1))
        target = model._default_manager.using(using).get(pk=object_pk)
    except TypeError:
        return CommentPostBadRequest(
            "Invalid content_type value: %r" % escape(ctype))
    except AttributeError:
        return CommentPostBadRequest(
            "The given content-type %r does not resolve to a valid model." % \
                escape(ctype))
    except ObjectDoesNotExist:
        return CommentPostBadRequest(
            "No object matching content-type %r and object PK %r exists." % \
                (escape(ctype), escape(object_pk)))
    except (ValueError, ValidationError) as e:
        return CommentPostBadRequest(
            "Attempting go get content-type %r and object PK %r exists raised %s" % \
                (escape(ctype), escape(object_pk), e.__class__.__name__))

    # Construct the comment form
    form = comments.get_form()(target, data=data)

    # Check security information
    if form.security_errors():
        return CommentPostBadRequest(
            "The comment form failed security verification: %s" % \
                escape(str(form.security_errors())))

    # Check for next
    if not next:
        next = data.get("next")

    # If there are errors show the comment
    if form.errors:
        template_list = [
            "comments/%s/%s/form.html" % (model._meta.app_label, model._meta.module_name),
            "comments/%s/form.html" % model._meta.app_label,
            "comments/form.html",
        ]
        return render_to_response(
            template_list, {
                "comment": form.data.get("comment", ""),
                "form": form,
                "next": data.get("next", next),
            },
            RequestContext(request, {})
        )

    # Otherwise create the comment
    comment = form.get_comment_object()
    comment.ip_address = request.META.get("REMOTE_ADDR", None)
    if request.user.is_authenticated:
        comment.user = request.user

    # Signal that the comment is about to be saved
    responses = signals.comment_will_be_posted.send(
        sender=comment.__class__,
        comment=comment,
        request=request
    )

    for (receiver, response) in responses:
        if response == False:
            return CommentPostBadRequest(
                "comment_will_be_posted receiver %r killed the comment" % receiver.__name__)

    # Save the comment and signal that it was saved
    comment.save()
    signals.comment_was_posted.send(
        sender=comment.__class__,
        comment=comment,
        request=request
    )
    messages.success(request, 'Your comment was saved.')
    return redirect(next)