Stack Basics
============

Calc uses RPN notation.  If you are not familar with RPN, *Note RPN
Tutorial::.

To add the numbers 1 and 2 in Calc you would type the keys: `1 RET 2
+'.  (RET corresponds to the ENTER key on most calculators.)  The
first three keystrokes "push" the numbers 1 and 2 onto the stack.  The
`+' key always "pops" the top two numbers from the stack, adds them,
and pushes the result (3) back onto the stack.  This number is ready
for further calculations: `5 -' pushes 5 onto the stack, then pops the
3 and 5, subtracts them, and pushes the result (-2).

Note that the "top" of the stack actually appears at the *bottom*
of the buffer.  A line containing a single `.' character signifies
the end of the buffer; Calculator commands operate on the number(s)
directly above this line.  The `d t' (`calc-truncate-stack')
command allows you to move the `.' marker up and down in the stack;
See Truncating the Stack.

Stack elements are numbered consecutively, with number 1 being the top
of the stack.  These line numbers are ordinarily displayed on the
lefthand side of the window.  The `d l' (`calc-line-numbering')
command controls whether these numbers appear.  (Line numbers may be
turned off since they slow the Calculator down a bit and also clutter
the display.)

The unshifted letter `o' (`calc-realign') command repositions the
cursor to its top-of-stack "home" position.  It also undoes any
horizontal scrolling in the window.  If you give it a numeric prefix
argument, it instead moves the cursor to the specified stack element.

The RET (or equivalent SPC) key is only required to separate two
consecutive numbers.  (After all, if you typed `1 2' by themselves the
Calculator would enter the number 12.)  If you press `RET' or `SPC'
*not* right after typing a number, the key duplicates the number on
the top of the stack.  `RET *' is thus a handy way to square a number.

The DEL key pops and throws away the top number on the stack.
The TAB key swaps the top two objects on the stack.
See Stack and Trail, for descriptions of these and other stack-related
commands.