theorem Th26:
  a <= b & f is_integrable_on ['a,b'] & f| ['a,b'] is bounded &
  ['a,b'] c= dom f & c in ['a,b'] & d in ['a,b']
  implies integral(-f,c,d) = -integral(f,c,d)
  proof
    -f = (-1)(#)f by NFCONT_4:7;
    hence thesis by Th25;
  end;
