theorem Th58:
  X1 c= Y1 & X2 c= Y2 & X3 c= Y3 implies [:X1,X2,X3:] c= [:Y1,Y2,
  Y3:]
proof
A1: [:X1,X2,X3:] = [:[:X1,X2:],X3:] & [:Y1,Y2,Y3:] = [:[:Y1,Y2:],Y3:] by
ZFMISC_1:def 3;
  assume X1 c= Y1 & X2 c= Y2;
  then
A2: [:X1,X2:] c= [:Y1,Y2:] by ZFMISC_1:96;
  assume X3 c= Y3;
  hence thesis by A2,A1,ZFMISC_1:96;
end;
