code: func (d *Descriptor) SetValue(b []byte) {
  	if d.ReadHandler != nil {
  		panic("descriptor has been configured with a read handler")
  	}
  	d.Property |= CharRead
  	d.Value = make([]byte, len(b))
  	copy(d.Value, b)
  }
docs: // SetValue makes the descriptor support read requests, and returns a static value.
  // SetValue must be called before the containing service is added to a server.
  // SetValue panics if the descriptor has already configured with a ReadHandler.