code: func (bb *Builder) PartsSize() int64 {
  	n := int64(0)
  	if parts, ok := bb.m["parts"].([]BytesPart); ok {
  		for _, part := range parts {
  			n += int64(part.Size)
  		}
  	}
  	return n
  }
docs: // PartsSize returns the number of bytes represented by the "parts" field.