let x, y be Element of [:REAL,REAL:]; :: thesis: taxi_dist2 . (x,y) = taxi_dist2 . (y,x)
reconsider x1 = x `1 , x2 = x `2 , y1 = y `1 , y2 = y `2 as Element of REAL ;
A1: ( x = [x1,x2] & y = [y1,y2] ) ;
then taxi_dist2 . (x,y) = (real_dist . (x1,y1)) + (real_dist . (x2,y2)) by Def16
.= (real_dist . (y1,x1)) + (real_dist . (x2,y2)) by METRIC_1:9
.= (real_dist . (y1,x1)) + (real_dist . (y2,x2)) by METRIC_1:9
.= taxi_dist2 . (y,x) by A1, Def16 ;
hence taxi_dist2 . (x,y) = taxi_dist2 . (y,x) ; :: thesis: verum