theorem Th21: :: BORSUK_1:21

for X, Y being non empty TopSpace
for H being Subset-Family of [:X,Y:]
for X1 being Subset of X
for Y1 being Subset of Y st H is Cover of [:X1,Y1:] holds
( ( Y1 <> {} implies (Pr1 (X,Y)) .: H is Cover of X1 ) & ( X1 <> {} implies (Pr2 (X,Y)) .: H is Cover of Y1 ) )