> For utf16, (arguably) a missing BOM should merely assume machian
endianess.
> For utf_16_le, utf_16_be input, both should accept & discard a BOM.
> On output, I'm not sure; maybe all should write a BOM unless passed a flag
> signifying no bom?
> Or to preserve backward compat, could have a parm write_bom defaulting to
> True for utf16 and False for utf_16_le and utf_16_be. This is a 
> modification of the originial request (for a force_bom flag).

The Unicode FAQ (http://unicode.org/faq/utf_bom.html#28) clearly states:

"""
Q: How I should deal with BOMs?
[...]
Where the precise type of the data stream is known (e.g. Unicode
big-endian or Unicode little-endian), the BOM should not be used. In
particular, whenever a data stream is declared to be UTF-16BE, UTF-16LE,
UTF-32BE or UTF-32LE a BOM *must* not be used. [...]