theorem Th27: :: ARMSTRNG:27

for X being set
for F being Dependency-set of X
for A, B being Subset of X holds
( A ^|^ B,F iff ( [A,B] in F & ( for A9, B9 being Subset of X holds
( not [A9,B9] in F or ( not A <> A9 & not B <> B9 ) or not [A,B] <= [A9,B9] ) ) ) )