reserve X,Y,x,y for set;
reserve A for non empty Poset;
reserve a,a1,a2,a3,b,c for Element of A;
reserve S,T for Subset of A;
reserve f for Choice_Function of BOOL(the carrier of A);
reserve fC,fC1,fC2 for Chain of f;
reserve R for Relation,
  A for non empty Poset,
  C for Chain of A,
  S for Subset of A,
  a,a1,a2,b,c1,c2 for Element of A;

theorem
  a is_inferior_of the InternalRel of A iff for b st a <> b holds a < b
proof
A1: the carrier of A = field(the InternalRel of A) by ORDERS_1:15;
  thus a is_inferior_of the InternalRel of A implies for b st a <> b holds a <
  b
  proof
    assume
A2: a is_inferior_of the InternalRel of A;
    let b;
    assume
A3: a <> b;
    then [a,b] in the InternalRel of A by A1,A2;
    then a <= b;
    hence thesis by A3;
  end;
  assume
A4: for b st a <> b holds a < b;
  thus a in field(the InternalRel of A) by A1;
  let y;
  assume y in field(the InternalRel of A);
  then reconsider b = y as Element of A by ORDERS_1:15;
  assume y <> a;
  then a < b by A4;
  then a <= b;
  hence thesis;
end;
