def _coeff_isneg(a):
    """Return True if the leading Number is negative.

    Examples
    ========

    >>> _coeff_isneg(-3*pi)
    True
    >>> _coeff_isneg(Integer(3))
    False
    >>> _coeff_isneg(-oo)
    True
    >>> _coeff_isneg(Symbol('n', negative=True))  # coeff is 1
    False

    """
    if a.is_Mul or a.is_MatMul:
        a = a.args[0]
    return a.is_Number and a.is_negative
