code: func (d *Decoder) mustgetc() (b byte, ok bool) {
  	if b, ok = d.getc(); !ok {
  		if d.err == io.EOF {
  			d.err = d.syntaxError("unexpected EOF")
  		}
  	}
  	return
  }
docs: // Must read a single byte.
  // If there is no byte to read,
  // set d.err to SyntaxError("unexpected EOF")
  // and return ok==false