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Complete this python program:
def constant_renumber(expr, symbolname, startnumber, endnumber):
assistant
#Here is the Python program you requested.

    """Renumber arbitrary constants in ``expr`` to have numbers 1 through `N`
    where `N` is ``endnumber - startnumber + 1`` at most.
    In the process, this reorders expression terms in a standard way.
    This is a simple function that goes through and renumbers any
    :py:class:`~sympy.core.symbol.Symbol` with a name in the form ``symbolname
    + num`` where ``num`` is in the range from ``startnumber`` to
    ``endnumber``.
    Symbols are renumbered based on ``.sort_key()``, so they should be
    numbered roughly in the order that they appear in the final, printed
    expression.  Note that this ordering is based in part on hashes, so it can
    produce different results on different machines.
    The structure of this function is very similar to that of
    :py:meth:`~sympy.solvers.ode.constantsimp`.
    Examples
    >>> from sympy import symbols, Eq, pprint
    >>> from sympy.solvers.ode import constant_renumber
    >>> x, C0, C1, C2, C3, C4 = symbols(\x,C:5\)
    Only constants in the given range (inclusive) are renumbered;
    the renumbering always starts from 1:
    >>> constant_renumber(C1 + C3 + C4, \C\, 1, 3)
    C1 + C2 + C4
    >>> constant_renumber(C0 + C1 + C3 + C4, \C\, 2, 4)
    C0 + 2*C1 + C2
    >>> constant_renumber(C0 + 2*C1 + C2, \C\, 0, 1)
    C1 + 3*C2
    >>> pprint(C2 + C1*x + C3*x**2)
    2
    C1*x + C2 + C3*x
    >>> pprint(constant_renumber(C2 + C1*x + C3*x**2, \C\, 1, 3))
    2
    C1 + C2*x + C3*x
    """
    if (type(expr) in (set, list, tuple)):
        return type(expr)([constant_renumber(i, symbolname=symbolname, startnumber=startnumber, endnumber=endnumber) for i in expr])
    global newstartnumber
    newstartnumber = 1
    constants_found = ([None] * (endnumber + 2))
    constantsymbols = [Symbol((symbolname + ('%d' % t))) for t in range(startnumber, (endnumber + 1))]
    C_1 = [(ci, S.One) for ci in constantsymbols]
    sort_key = (lambda arg: default_sort_key(arg.subs(C_1)))
    def _constant_renumber(expr):
        '\n                                We    need    to    have    an    internal    recursive    function    so    that\n                                newstartnumber    maintains    its    values    throughout    recursive    calls.\n\n                                '
        global newstartnumber
        if isinstance(expr, Equality):
            return Eq(_constant_renumber(expr.lhs), _constant_renumber(expr.rhs))
        if ((type(expr) not in (Mul, Add, Pow)) and (not expr.is_Function) and (not expr.has(*constantsymbols))):
            return expr
        elif expr.is_Piecewise:
            return expr
        elif (expr in constantsymbols):
            if (expr not in constants_found):
                constants_found[newstartnumber] = expr
                newstartnumber += 1
            return expr
        elif (expr.is_Function or expr.is_Pow or isinstance(expr, Tuple)):
            return expr.func(*[_constant_renumber(x) for x in expr.args])
        else:
            sortedargs = list(expr.args)
            sortedargs.sort(key=sort_key)
            return expr.func(*[_constant_renumber(x) for x in sortedargs])
    expr = _constant_renumber(expr)
    newconsts = symbols(('C1:%d' % newstartnumber))
    expr = expr.subs(zip(constants_found[1:], newconsts), simultaneous=True)
    return expr