:: deftheorem defines TOP COMBGRAS:def 5 :
for k being Nat
for X being non empty set
for T being Subset of the Points of (G_ (k,X)) holds
( T is TOP iff ex S being Subset of X st
( card S = k + 1 & T = { A where A is Subset of X : ( card A = k & A c= S ) } ) );