code: func (f *fieldVal) Equals(val *fieldVal) bool {
  	// Xor only sets bits when they are different, so the two field values
  	// can only be the same if no bits are set after xoring each word.
  	// This is a constant time implementation.
  	bits := (f.n[0] ^ val.n[0]) | (f.n[1] ^ val.n[1]) | (f.n[2] ^ val.n[2]) |
  		(f.n[3] ^ val.n[3]) | (f.n[4] ^ val.n[4]) | (f.n[5] ^ val.n[5]) |
  		(f.n[6] ^ val.n[6]) | (f.n[7] ^ val.n[7]) | (f.n[8] ^ val.n[8]) |
  		(f.n[9] ^ val.n[9])
  
  	return bits == 0
  }
docs: // Equals returns whether or not the two field values are the same.  Both
  // field values being compared must be normalized for this function to return
  // the correct result.