theorem
  a in [:X1,X2:] implies ex x1,x2 st a=[x1,x2]
proof
  assume a in [:X1,X2:];
  then consider x1,x2 being object such that
A1: x1 in X1 and
A2: x2 in X2 and
A3: a=[x1,x2] by ZFMISC_1:def 2;
  reconsider x2 as Element of X2 by A2;
  reconsider x1 as Element of X1 by A1;
  take x1,x2;
  thus thesis by A3;
end;
