old: original text
start: starting byte position
num: number of bytes to be replaced
new: replacement string
REPLACEB replaces the string of valid unicode characters starting at the byte start and ending at start+num-1 with the string new.
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.