consider x1 being set such that
A1: x1 in X1 by XBOOLE_0:def 1;
consider x2 being set such that
A2: x2 in X2 by XBOOLE_0:def 1;
[x1,x2] in [:X1,X2:] by A1, A2, ZFMISC_1:def 2;
hence not [:X1,X2:] is empty ; :: thesis: verum