theorem
  <*r1*> - <*r2*> = <*r1-r2*>
proof
  reconsider s1=r1, s2=r2 as Element of REAL by XREAL_0:def 1;
  <*s1*> - <*s2*> = <*diffreal.(s1,s2)*> by FINSEQ_2:74
    .= <*r1-r2*> by BINOP_2:def 10;
  hence thesis;
end;
