defpred S₁[ Element of M] means $1 is_between p,r;
{ q where q is Element of M : S₁[q] } c= the carrier of M from FRAENKEL:sch 10();
hence { q where q is Element of M : q is_between p,r } \/ {p,r} is Subset of M by XBOOLE_1:8; :: thesis: verum