Defining New Simple Commands
----------------------------

If a `defmath' form contains an `interactive' clause, it defines a
Calculator command.  Actually such a `defmath' results in *two*
function definitions: One, a `calcFunc-' function as was just
described, with the `interactive' clause removed.  Two, a `calc-'
function with a suitable `interactive' clause and some sort of wrapper
to make the command work in the Calc environment.

In the simple case, the `interactive' clause has the same form as for
normal Emacs Lisp commands:

     (defmath increase-precision (delta)
       "Increase precision by DELTA."     ; This is the "documentation string"
       (interactive "p")                  ; Register this as a M-x-able command
       (setq calc-internal-prec (+ calc-internal-prec delta)))

This expands to the pair of definitions,

     (defun calc-increase-precision (delta)
       "Increase precision by DELTA."
       (interactive "p")
       (calc-wrapper
        (setq calc-internal-prec (math-add calc-internal-prec delta))))

     (defun calcFunc-increase-precision (delta)
       "Increase precision by DELTA."
       (setq calc-internal-prec (math-add calc-internal-prec delta)))

where in this case the latter function would never really be used!
Note that since the Calculator stores small integers as plain Lisp
integers, the `math-add' function will work just as well as the native
`+' even when the intent is to operate on native Lisp integers.

The `calc-wrapper' call invokes a macro which surrounds the body of
the function with code that looks roughly like this:

     (let ((calc-command-flags nil))
       (unwind-protect
           (save-excursion
             (calc-select-buffer)
             *body of function*
             *renumber stack*
             *clear* Working *message*)
         *realign cursor and window*
         *clear Inverse, Hyperbolic, and Keep Args flags*
         *update Emacs mode line*))

The `calc-select-buffer' function selects the `*Calculator*' buffer if
necessary, say, because the command was invoked from inside the `*Calc
Trail*' window.

You can call, for example, `(calc-set-command-flag 'no-align)' to set
the above-mentioned command flags.  The following command flags are
recognized by Calc routines:

`renum-stack'
     Stack line numbers `1:', `2:', and so on must be renumbered after
     this command completes.  This is set by routines like
     `calc-push'.

`clear-message'
     Calc should call `(message "")' if this command completes
     normally (to clear a "Working..." message out of the echo area).

`no-align'
     Do not move the cursor back to the `.' top-of-stack marker.

`position-point'
     Use the variables `calc-position-point-line' and
     `calc-position-point-column' to position the cursor after this
     command finishes.

`keep-flags'
     Do not clear `calc-inverse-flag', `calc-hyperbolic-flag', and
     `calc-keep-args-flag' at the end of this command.

`do-edit'
     Switch to buffer `*Calc Edit*' after this command.

`hold-trail'
     Do not move trail pointer to end of trail when something is
     recorded there.

Calc reserves a special prefix key, shift-`Y', for user-written
extensions to Calc.  There are no built-in commands that work with
this prefix key; you must call `define-key' from Lisp (probably from
inside a `calc-define' property) to add to it.  Initially only `Y ?' 
is defined; it takes help messages from a list of strings (initially
`nil') in the variable `calc-Y-help-msgs'.  All other undefined keys
except for `Y' are reserved for use by future versions of Calc.

If you are writing a Calc enhancement which you expect to give to
others, it is best to minimize the number of `Y'-key sequences you
use.  In fact, if you have more than one key sequence you should
consider defining three-key sequences with a `Y', then a key that
stands for your package, then a third key for the particular command
within your package.

Users may wish to install several Calc enhancements, and it is
possible that several enhancements will choose to use the same key.
In the example below, a variable `inc-prec-base-key' has been defined
to contain the key that identifies the `inc-prec' package.  Its value
is initially `"P"', but a user can change this variable if necessary
without having to modify the file.

Here is a complete file, `inc-prec.el', which makes a `Y P I' command
that increases the precision, and a `Y P D' command that decreases the
precision.

     ;;; Increase and decrease Calc precision.  Dave Gillespie, 5/31/91.
     ;;; (Include copyright or copyleft stuff here.)

     (defvar inc-prec-base-key "P"
       "Base key for inc-prec.el commands.")

     (put 'calc-define 'inc-prec '(progn

     (define-key calc-mode-map (format "Y%sI" inc-prec-base-key)
                 'increase-precision)
     (define-key calc-mode-map (format "Y%sD" inc-prec-base-key)
                 'decrease-precision)

     (setq calc-Y-help-msgs
           (cons (format "%s + Inc-prec, Dec-prec" inc-prec-base-key)
                 calc-Y-help-msgs))

     (defmath increase-precision (delta)
       "Increase precision by DELTA."
       (interactive "p")
       (setq calc-internal-prec (+ calc-internal-prec delta)))

     (defmath decrease-precision (delta)
       "Decrease precision by DELTA."
       (interactive "p")
       (setq calc-internal-prec (- calc-internal-prec delta)))

     ))  ; end of calc-define property

     (run-hooks 'calc-check-defines)