Pathname identifying location of buffer .
An external format spec.
A buffer whose contents are examined.
Length (an integer) up to which buffer should be examined.
A new external format spec created by merging ef-spec with the encoding that was found.
When called as part of open's encoding detection routine, if byte pair FE FF is found at the start of the file, it is assumed to be UTF16-BE encoded. This encoding is represented by the ef-spec
(:unicode :little-endian nil)