theorem I1part2Reverse:
  a <> b & b <> y & y on_line a,b
    implies a,b equal_line y,b
  proof
     assume
H1:  a <> b & b <> y & y on_line a,b; then
Y1:  a,b equal_line b,a & b,y equal_line y,b by LineEqA1; then
     b,a equal_line b,y by H1, I1part2; then
     a,b equal_line b,y by Y1;
     hence thesis by Y1;
   end;
