theorem Th94:
  trap a,b,c,d,d implies d=b
proof
  assume
A1: trap a,b,c,d,d;
  then a,c // b,d;
  then
A2: d,b // a,c by Th6;
  d,a,b are_collinear by A1;
  then d,a // d,b;
  then
A3: d,b // a,d by Th6;
  assume not thesis;
  then a,d // a,c by A3,A2,Def1;
  then
A4: d,a // d,c by Th7;
  not d,a,c are_collinear by A1;
  hence contradiction by A4;
end;
