[]*]     # What about this?!  valid
[:*096]  # invalid: 096 isn't a valid octal number
a [:*0]  # as many colons as string1 was long (not to be confused
	 # with a character class)
[:*]     # ditto
[:*016]  # 14 colons
[=]=]    # valid: equivalence class containing ']'
[-a      # valid, assuming `[' is before 'a' in collating sequence
 -]      # valid, assuming ` ' is before ']' in collating sequence
--]      # valid, assuming `-' is before ']' in collating sequence
\0-\377  # valid
[\0-\377]# valid, (but brackets will be mapped to corresponding chars
	 # in other string)
abcde[:* # valid, but none of the characters is considered special
abc xyzdef # Should this (str2 longer than str1) evoke a warning?
           # Probably so if we're only translating, but if also deleting or
	   # squeezing this makes sense.
abcdef : # Map abcdef all to `:', as if str2 had been [:*]