def is_strictly_decreasing(f, interval=S.Reals, symbol=None):
    """
    Returns if a function is strictly decreasing or not, in the given
    ``Interval``.

    Examples
    ========

    >>> from sympy import is_strictly_decreasing
    >>> from sympy.abc import x, y
    >>> from sympy import S, Interval, oo
    >>> is_strictly_decreasing(1/(x**2 - 3*x), Interval.open(1.5, 3))
    True
    >>> is_strictly_decreasing(1/(x**2 - 3*x), Interval.Lopen(3, oo))
    True
    >>> is_strictly_decreasing(1/(x**2 - 3*x), Interval.Ropen(-oo, S(3)/2))
    False
    >>> is_strictly_decreasing(-x**2, Interval(-oo, 0))
    False
    >>> is_strictly_decreasing(-x**2 + y, Interval(-oo, 0), x)
    False

    """
    f = sympify(f)
    free_sym = f.free_symbols

    if symbol is None:
        if len(free_sym) > 1:
            raise NotImplementedError('is_strictly_decreasing has not yet been implemented '
                                        'for all multivariate expressions')
        elif len(free_sym) == 0:
            return False
        symbol = free_sym.pop()

    df = f.diff(symbol)
    df_neg_interval = solveset(0 < df, symbol, domain=S.Reals)
    return interval.is_subset(df_neg_interval)
dataset/ETHPy150Open sympy/sympy/sympy/calculus/singularities.py/is_strictly_decreasing/swapped operands of `<`