theorem Th94:
  A**--B = --(A**B)
proof
  let a;
  hereby
    assume a in A**--B;
    then consider c,c1 such that
A1: a = c*c1 and
A2: c in A and
A3: c1 in --B;
    -c1 in B by A3,Th12;
    then
A4: c*-c1 in A**B by A2;
    a = -(c*-c1) by A1;
    hence a in --(A**B) by A4;
  end;
  assume a in --(A**B);
  then -a in A**B by Th12;
  then consider c,c1 such that
A5: -a = c*c1 and
A6: c in A and
A7: c1 in B;
  a = c*-c1 & -c1 in --B by A5,A7;
  hence thesis by A6;
end;
