Newmark b method for bi-linear SDOF structure with iteration

The original program was developed by Mr. Christopher Wong. Thanks for his compact work. I learned a lot from the original program. The reference book in my case was Dynamics of Structures by Dr. Chopra, second ed.2001, p191. The model parameters seems to be different between 2nd and 4th edition. I tried to reproduce Table E5.6, p192. The convergence procedure in the original program was not clear. I am a beginner of Matlab. So, for the sakes of solving non-linear DSOF and practicing Matlab, I started to modify the original program of Mr. Christopher Wong. The followings are items of modification.

1) Parameters used are from 2nd edition of Dynamics of Structures by Dr. Chopra.
2) Full wave length of sine wave was used to generate yielding in both directions.
3) Compatibility conditions were considered to calculate acceleration.
4) A variable flg was introduced to specify a state of force-deformation relation, such as flg=1 elastic, flg=2 yield, flg=3 rebound elastic.
5) Rebound elastic is generated when du*du0<0 of flg=2, where du is incremental deformation.