theorem
  a is terminal & b is terminal implies a,b are_isomorphic
proof
  assume that
A1: a is terminal and
A2: b is terminal;
  set g = the Morphism of b,a;
  set f = the Morphism of a,b;
A3: Hom(a,b) <> {} by A2;
  take f;
  now
    thus Hom(b,a) <> {} by A1;
    take g;
    thus f*g = id b & g*f = id a by A1,A2,Th49;
  end;
  hence thesis by A3;
end;
