'Define an ascii string at ad (null terminated).
returns True if ok'

def set_ascii(self, ad):

    ad2 = self.mem.get_head_addr(ad)
    sz = self.__binary.is_string(ad, min_bytes=1)
    if (not sz):
        return False
    if (not self.__undefine(ad2)):
        return False
    self.mem.add(ad, sz, MEM_ASCII)
    return True
