theorem :: TOPREALA:39

for a, b, r, s being Real st a <= b & r <= s holds
for A being open Subset of (Closed-Interval-TSpace (a,b))
for B being open Subset of (Closed-Interval-TSpace (r,s)) holds product ((1,2) --> (A,B)) is open Subset of (Trectangle (a,b,r,s))