unctive normal form.

    Examples
    ========

    >>> from sympy.logic.boolalg import is_dnf
    >>> from sympy.abc import A, B, C
    >>> is_dnf(A | B | C)
    True
    >>> is_dnf(A & B & C)
    True
    >>> is_dnf((A & B) | C)
    True
    >>> is_dnf(A & (B | C))
    False

    N)