let a, b, c, d be Real; :: thesis: ( a <= b & c <= d implies NW-corner (rectangle (a,b,c,d)) = |[a,d]| )
assume that
A1: a <= b and
A2: c <= d ; :: thesis: NW-corner (rectangle (a,b,c,d)) = |[a,d]|
set K = rectangle (a,b,c,d);
A3: NW-corner (rectangle (a,b,c,d)) = |[(W-bound (rectangle (a,b,c,d))),(N-bound (rectangle (a,b,c,d)))]| by PSCOMP_1:def 12;
W-bound (rectangle (a,b,c,d)) = a by A1, A2, Th36;
hence NW-corner (rectangle (a,b,c,d)) = |[a,d]| by A1, A2, A3, Th37; :: thesis: verum