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omc-context.lsp
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Manage function execution context. Automates the assignment and
resetting of system variables, error catching and grouping calls to
*command* for undoing between the enter and exit functions.

To use it, follow these steps:

1. Define variables *\*error\** and *context* as local to the
   function;
2. Call ``omc-context-enter`` with an association list of system
   variables and their values desired during the execution of the
   function: ``(omc-context-enter '((var1 . value1) (var2 . value2)
   ... )))``. Use ``(omc-context-enter nil)`` if you don't need any.
3. Continue the function code normally.
4. Call ``omc-context-exit`` to reset the system variables to their
   previous values and ends the undo grouping. If it is the last call
   in a function, it is not necessary to call ``princ`` to exit
   quietly.

If you need to process errors somehow, define the ``*error*`` function
after ``omc-context-enter``, but keep in mind the procedures in
``omc-context-error`` to avoid inconsistencies with the context
management functions.

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Functions
*********

(**omc\-context\-enter** *options*)

   Enter function execution context: set system variables, start undo group and define error catcher.

(**omc\-context\-exit**)

   Exit function execution context: reset variables to previous values and end undo group.

(**omc\-context\-error** *msg*)

   Catch errors and exit function execution context: reset system variables to previous values and end undo group.

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Global variables
****************

* **\*error\*** assigned in

   * *omc\-context\-enter*

* **\*omc\-context\-undo\-stack\-size\*** assigned in

   * *Top level*
   * *omc\-context\-enter*
   * *omc\-context\-error*
   * *omc\-context\-exit*

* **context** assigned in

   * *omc\-context\-enter*

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Functions called on load
************************

* **princ**