Tkinter: Entry, how to get INSERTpos?

[skanemupp at yahoo.se]

in this function i want to be able to use the cursor and delete in the
middle of a number.
how do i get the value of anchor or insert?
i want to write:
e.delete(pos-1,pos)

def Backspace():
    a=len(e.get())
    e.delete(a-1,END)
    #e.delete(INSERT, END)
    #e.delete(ANCHOR,END)

the complete program:

from __future__ import division
import Tkinter
from Tkinter import *

mygui = Tkinter.Tk()

mygui.title("Calculator")

l = Label(mygui, text="Answer: ")
l.place(relx=0.15, rely=0.2, anchor=CENTER)

e = Entry(mygui)
e.place(relx=0.48, rely=0.1, anchor=CENTER, width=173)

c = Entry(mygui)
c.place(relx=0.6, rely=0.2, anchor=CENTER)

def Disp(nstr):
    e.insert(INSERT, nstr)

def Calc():
    expr=e.get()
    c.delete(0, END)
    try:
        c.insert(END, eval(expr))
    except:
        c.insert(END, "Not computable")

def Erase():
    e.delete(0,END)
    c.delete(0, END)

def Backspace():
    a=len(e.get())
    e.delete(a-1,END)
    #e.delete(INSERT, END)
    #e.delete(ANCHOR,END)


x = 0.1
y = 0.4
for char in '123+456-789*0()/.':
    b = Button(mygui, text=char, command=lambda n=char:Disp(n),
width=2, height=1)
    b.place(relx=x, rely=y, anchor=CENTER)
    x=x+0.1
    if x==0.5:
        x=0.1
        y=y+0.1

b = Button(mygui, text="^", command=lambda n="**":Disp(n), width=2,
height=1)
b.place(relx=0.2, rely=0.8, anchor=CENTER)
b = Button(mygui, text="C",command=Erase, width=2, height=1)
b.place(relx=0.3, rely=0.8, anchor=CENTER)
b = Button(mygui, text="c",command=Backspace, width=2, height=1)
b.place(relx=0.4, rely=0.8, anchor=CENTER)
b = Button(mygui, text="=",command=Calc, width=12, height=1)
b.place(relx=0.25, rely=0.9, anchor=CENTER)

mygui.mainloop()