theorem Th24:
  Mid a,b,c & Mid a,b9,c9 & MDist b,b9 implies c = c9
proof
  assume that
A1: Mid a,b,c and
A2: Mid a,b9,c9 and
A3: MDist b,b9;
  Mid a,b9,c by A1,A3,Th23;
  hence thesis by A2,Th22;
end;
