theorem Th61:
  t |= f 'or' g iff (t|= f or t|= g)
proof
  t |= f 'or' g iff t|= 'not' ('not'(f) '&' 'not'(g)) by Def54;
  then t |= f 'or' g iff not t|= 'not'(f) '&' 'not'(g) by Th57;
  then t |= f 'or' g iff not t|= 'not'(f) or not t|= 'not'(g) by Th58;
  hence thesis by Th57;
end;
