code: func (f *fieldVal) IsZero() bool {
  	// The value can only be zero if no bits are set in any of the words.
  	// This is a constant time implementation.
  	bits := f.n[0] | f.n[1] | f.n[2] | f.n[3] | f.n[4] |
  		f.n[5] | f.n[6] | f.n[7] | f.n[8] | f.n[9]
  
  	return bits == 0
  }
docs: // IsZero returns whether or not the field value is equal to zero.