Changed to:

    # _is_even and _is_odd are used for correct evaluation of csc(-x), sec(-x)
    # TODO refactor into TrigonometricFunction common parts of
    # trigonometric functions eval() like even/odd, func(x+2*k*pi), etc.
    _is_even = None  # optional, to be defined in subclass
    _is_odd = None   # optional, to be defined in subclass