reserve S for non empty satisfying_CongruenceIdentity
              satisfying_SegmentConstruction
              satisfying_BetweennessIdentity
              satisfying_Pasch
              TarskiGeometryStruct;
reserve a,b for POINT of S;
reserve A for Subset of S;
reserve S for non empty satisfying_Tarski-model
              TarskiGeometryStruct;
reserve a,b,c,m,r,s for POINT of S;
reserve A for Subset of S;
reserve S         for non empty satisfying_Lower_Dimension_Axiom
                                satisfying_Tarski-model
                                TarskiGeometryStruct,
        a,b,c,d,m,p,q,r,s,x for POINT of S,
        A,A9,E              for Subset of S;

theorem Th23:
  A is_line & not a in A & not b in A &
  b in half-plane(A,a) implies half-plane(A,b) = half-plane(A,a)
  proof
    assume that
A1: A is_line and
A2: not a in A and
A3: not b in A and
A4: b in half-plane(A,a);
    a in half-plane(A,b) by A1,A2,A3,A4,Th21;
    hence thesis by A4,Th22;
  end;
