reserve a,b,c,x,y,z for object,X,Y,Z for set,
  n for Nat,
  i,j for Integer,
  r,r1,r2,r3,s for Real,
  c1,c2 for Complex,
  p for Point of TOP-REAL n;

theorem Th30:
  -c[100] = c[-100]
  proof
    -c100 = |[-1,-Q,-Q]| by EUCLID_5:11
    .= c[-100];
    hence thesis;
  end;
