def even_perm_is_orientable(p):
    """
    Test whether the permutation is orientable (that is do not mix non-negatives with negatives)

    EXAMPLES::

        sage: from surface_dynamics.misc.even_permutation import even_perm_is_orientable
        sage: even_perm_is_orientable([1,0,-1,-2])
        True
        sage: even_perm_is_orientable([0,1,-2,-1])
        True
        sage: even_perm_is_orientable([1,-1,0,-2])
        False
    """
    n = len(p) // 2
    for i in range(n):
        if p[i] < 0:
            return False
    return True
