def assignment_source(num_pre, num_post, LISTNAME, ITERNAME):
    u"""
    Accepts num_pre and num_post, which are counts of values
    before and after the starg (not including the starg)
    Returns a source fit for Assign() from fixer_util
    """
    children = []
    pre = unicode(num_pre)
    post = unicode(num_post)
    # This code builds the assignment source from lib2to3 tree primitives.
    # It's not very readable, but it seems like the most correct way to do it.
    if num_pre > 0:
        pre_part = Node(syms.power, [Name(LISTNAME), Node(syms.trailer, [Leaf(token.LSQB, u"["), Node(syms.subscript, [Leaf(token.COLON, u":"), Number(pre)]), Leaf(token.RSQB, u"]")])])
        children.append(pre_part)
        children.append(Leaf(token.PLUS, u"+", prefix=u" "))
    main_part = Node(syms.power, [Leaf(token.LSQB, u"[", prefix=u" "), Name(LISTNAME), Node(syms.trailer, [Leaf(token.LSQB, u"["), Node(syms.subscript, [Number(pre) if num_pre > 0 else Leaf(1, u""), Leaf(token.COLON, u":"), Node(syms.factor, [Leaf(token.MINUS, u"-"), Number(post)]) if num_post > 0 else Leaf(1, u"")]), Leaf(token.RSQB, u"]"), Leaf(token.RSQB, u"]")])])
    children.append(main_part)
    if num_post > 0:
        children.append(Leaf(token.PLUS, u"+", prefix=u" "))
        post_part = Node(syms.power, [Name(LISTNAME, prefix=u" "), Node(syms.trailer, [Leaf(token.LSQB, u"["), Node(syms.subscript, [Node(syms.factor, [Leaf(token.MINUS, u"-"), Number(post)]), Leaf(token.COLON, u":")]), Leaf(token.RSQB, u"]")])])
        children.append(post_part)
    source = Node(syms.arith_expr, children)
    return source