theorem Th21: :: TEX_1:21

for D being non empty set
for d0 being Element of D st not D \ {d0} is empty holds
for A being Subset of (STS (D,d0)) holds
( ( A = {d0} implies ( A is closed & A is boundary ) ) & ( not A is empty & A is closed & A is boundary implies A = {d0} ) )