reserve A,B,C for Ordinal,
        o for object,
        x,y,z,t,r,l for Surreal,
        X,Y for set;

theorem Th25:
  for A,B be Ordinal st A c= B holds Triangle A c= Triangle B
proof
  let A,B be Ordinal such that A1: A c= B;
  let x,y be object such that A2: [x,y] in Triangle A;
  reconsider x,y as Element of Day A by A2,ZFMISC_1:87;
  born x (+) born y c= A by A2,Def5;
  then born x (+) born y c= B by A1,XBOOLE_1:1;
  hence thesis by Def5;
end;
