:: deftheorem Def8 defines Between URYSOHN1:def 8 :
for T being non empty TopSpace
for A, B being Subset of T st T is normal & A <> {} & A is open & B is open & Cl A c= B holds
for b₄ being Subset of T holds
( b₄ is Between of A,B iff ( b₄ <> {} & b₄ is open & Cl A c= b₄ & Cl b₄ c= B ) );