hdr_guard x = "__" ++ map hchar x
  where hchar x | isAlphaNum x = toUpper x
        hchar _ = '_'
hdr_guard x = "__" ++ map hchar x
  where hchar x | isAlphaNum x = toUpper x
        hchar _ = '_'
hdr_guard x = "__" ++ map hchar x
  where hchar x | isAlphaNum x = toUpper x
        hchar _ = '_'