// SplitFunc is the signature of the split function used to tokenize the
// input. The arguments are an initial substring of the remaining unprocessed
// data and a flag, atEOF, that reports whether the [Reader] has no more data
// to give. The return values are the number of bytes to advance the input
// and the next token to return to the user, if any, plus an error, if any.
//
// Scanning stops if the function returns an error, in which case some of