A list of functions which attempt to determine a coding system.
Each function in this list should be written to operate on the
current buffer, but should not modify it in any way. The buffer
will contain undecoded text of parts of the file. Each function
should take one argument, SIZE, which says how many characters
(starting from point) it should look at.
If one of these functions succeeds in determining a coding
system, it should return that coding system. Otherwise, it
should return nil.
If a file has a `coding:' tag, that takes precedence over these functions, so they won't be called at all.