Interactive Functions
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The functions described here are used in implementing interactive Calc
commands.  Note that this list is not exhaustive!  If there is an
existing command that behaves similarly to the one you want to define,
you may find helpful tricks by checking the source code for that
command.

 -- Function: calc-set-command-flag FLAG
     Set the command flag FLAG.  This is generally a Lisp symbol, but
     may in fact be anything.  The effect is to add FLAG to the list
     stored in the variable `calc-command-flags', unless it is already
     there.  See Defining Simple Commands.

 -- Function: calc-clear-command-flag FLAG
     If FLAG appears among the list of currently-set command flags,
     remove it from that list.

 -- Function: calc-record-undo REC
     Add the "undo record" REC to the list of steps to take if the
     current operation should need to be undone.  Stack push and pop
     functions automatically call `calc-record-undo', so the kinds of
     undo records you might need to create take the form `(set SYM
     VALUE)', which says that the Lisp variable SYM was changed and
     had previously contained VALUE; `(store VAR VALUE)' which says
     that the Calc variable VAR (a string which is the name of the
     symbol that contains the variable's value) was stored and its
     previous value was VALUE (either a Calc data object, or `nil' if
     the variable was previously void); or `(eval UNDO REDO ARGS
     ...)', which means that to undo requires calling the function
     `(UNDO ARGS ...)' and, if the undo is later redone, calling
     `(REDO ARGS ...)'.

 -- Function: calc-record-why MSG ARGS
     Record the error or warning message MSG, which is normally a
     string.  This message will be replayed if the user types `w'
     (`calc-why'); if the message string begins with a `*', it is
     considered important enough to display even if the user doesn't
     type `w'.  If one or more ARGS are present, the displayed message
     will be of the form, `MSG: ARG1, ARG2, ...', where the arguments
     are formatted on the assumption that they are either strings or
     Calc objects of some sort.  If MSG is a symbol, it is the name of
     a Calc predicate (such as `integerp' or `numvecp') which the
     arguments did not satisfy; it is expanded to a suitable string
     such as "Expected an integer."  The `reject-arg' function calls
     `calc-record-why' automatically; See Predicates.

 -- Function: calc-is-inverse
     This predicate returns true if the current command is inverse,
     i.e., if the Inverse (`I' key) flag was set.

 -- Function: calc-is-hyperbolic
     This predicate is the analogous function for the `H' key.