s: the string
start_pos: the number of the byte with which to start (defaults to 1)
num_bytes: the maximum number of bytes to return (defaults to 1)
The semantics of this function is subject to change as various applications implement it.
While this function is syntactically Excel compatible, the differences in the underlying text encoding will usually yield different results.
While this function is OpenFormula compatible, most of its behavior is, at this time, implementation specific.