lec4 update

...tors/.ipynb_checkpoints/lec_03_Operators-checkpoint.ipynb

...Gurmail_lecture_notes/03_Operators/lec_03_Operators.ipynb

....ipynb_checkpoints/lec_04_Vars_and_Exprs-checkpoint.ipynb

..._04_Vars_and_Exprs_template_Gurmail_lec1-checkpoint.ipynb

..._04_Vars_and_Exprs_template_Gurmail_lec2-checkpoint.ipynb

...cture_notes/04_Vars_and_Exprs/lec_04_Vars_and_Exprs.ipynb

...d_Exprs/lec_04_Vars_and_Exprs_template_Gurmail_lec1.ipynb

...d_Exprs/lec_04_Vars_and_Exprs_template_Gurmail_lec2.ipynb