def flag(i, com, xi, yi):
"""
Function register click on a checkbutton and performs command
:param i: flag number which corresponds to command number in list_cb
:param com: command name
:param xi: x coordinate of chk
:param yi: y coordinate of chk
"""
chk = Checkbutton(window1, variable=list_cb[i], command=com)
chk.focus()
chk.place(x=xi, y=yi)
return
