def infer_cetacean_sex(f, m):
    """Infer WAStD sex from Cetacean db values.

    Arguments

    f String The db value of column "is female". Females are '1'.
    m String The db value of column "is male". Males are '1'.

    Return
    String One of WAStD SEX_CHOICES "na" (unknown sex), "female" or "male"
    """
    if f == "1":
        return "female"
    elif m == "1":
        return "male"
    else:
        return "na"