A1: 0 in ].(- (PI / 2)),(PI / 2).[ ;
A2: {0} c= ].(- (PI / 2)),(PI / 2).[ by A1, TARSKI:def 1;
].0,(PI / 2).[ c= ].(- (PI / 2)),(PI / 2).[ by XXREAL_1:46;
then ].0,(PI / 2).[ \/ {0} c= ].(- (PI / 2)),(PI / 2).[ by A2, XBOOLE_1:8;
hence [.0,(PI / 2).[ c= ].(- (PI / 2)),(PI / 2).[ by XXREAL_1:131; :: thesis: verum