:: deftheorem defines is_Bin_Op_Preserv REALSET1:def 3 :
for X being non trivial set
for F being BinOp of X
for x being Element of X holds
( F is_Bin_Op_Preserv x iff ( X \ {x} is Preserv of F & (F || X) \ {x} is BinOp of (X \ {x}) ) );