
theorem
  for x,y,z being Element of BOOLEAN holds nor3.<*x,y,z*>='not' (x 'or'
  y 'or' z) & nor3a.<*x,y,z*>='not' ('not' x 'or' y 'or' z) & nor3b.<*x,y,z*>=
'not' ('not' x 'or' 'not' y 'or' z) & and3.<*x,y,z*>='not' ('not' x 'or' 'not'
  y 'or' 'not' z) by Def17,Def18,Def19,Lm8;
