'almost' Emacs M-x calc (RPN) - RET push/dup, TAB swap, n negate, & 1/x, Q sqrt, U undo, DEL drop -
clearevo
--%*-Calc: 12 Deg (Calculator)
--%*- *Calc Trail*
7
8
9
/
C
4
5
6
*
DEL
1
2
3
-
swap
0
.
+/-
+
RET
√
x^y
1/x
ln
U undo