theorem Th71:
  dom NEx_Val(v,S,x,xSQ) = dom RestrictSub(x,All(x,S`1),xSQ)
proof
  rng (@RestrictSub(x,All(x,S`1),xSQ)) c= bound_QC-variables(Al);
  then rng (@RestrictSub(x,All(x,S`1),xSQ)) c= dom v by Th58;
  then dom NEx_Val(v,S,x,xSQ) = dom (@RestrictSub(x,All(x,S`1),xSQ)) by
RELAT_1:27;
  hence thesis by SUBSTUT1:def 2;
end;
