EXEC by John M O'Donnell (ODONNELL@LAMPF.LANL.GOV)          2-Oct-1994

[Note: EXEC is a command in the SYSUTILS library, also on this disk.
 If you use SYSUTILS, don't use this; you already have EXEC.  -jkh-]

--> S/SX/G/GX

EXEC adds path like functionality to the HP48.	This routine is
similar to ones written by Scott Burke and Darryl Okahata.  EXEC
searches a list of directories (given in the variables 'TRAIL' and
'TRAIL.SYS') for the first occurrence of an object named in stack
level 1.  At the end of the search, the object is evaluated in the
initial directory.

    Stack arguments:

	levels, N+1,...,2	- arguments required to evaluate
				  requested object.

	level		 1	- object to be found in another
				  directory and then evaluated.

    Other arguments:

	TRAIL.SYS		- an ordered list of directories to
				  search for the object named in level
				  2.  TRAIL.SYS is in HOME.

	TRAIL			- an ordered list of directories to
				  search for the object named in level
				  1.  This list is prepended to
				  TRAIL.SYS.  It is used to make
				  modifications to TRAIL.SYS local to
				  a given directory tree.  A default
				  TRAIL with no paths should be
				  included in HOME.  In the interest
				  of size and speed, little checking
				  is done - make sure TRAIL refers
				  only to existing directories.

    Return Stack:

	levels M,...,1		- results of evaluating the object.

    Error Messages:

	Bad Control Variable	- TRAIL, or TRAIL.SYS is not a list.

	Other messages are described in the HP48 manual.

    Discussion:

	When searching a directory for the requested object, only the
	specified directories (and not their parents) are searched.
	Thus, using the directory tree from Scott Burke's example:

				   HOME
				    /\
				WORK  PLAY
			       / / \	  \
			     W1 W2 W3	 GAMES

	and with TRAIL.SYS set to { { W2 } { WORK } }, the HOME
	directory would not be searched, and WORK would be searched
	only once.

	This version evaluates the requested object in the current
	directory, not the directory in which the object came from.
	(This can be easily changed.)

	I have considered points raised by the previous authors:

	(1) I do not require TRAIL or TRAIL.SYS to be in the HOME
	directory. It comes from the HP48 PATH, and a local copy is
	then maintained in the program.  This allows software packages
	to have their own path.

	(2) the name is short.

	(3) Using SYSRPL considerably speeded up the code.  Other
	speed improvements include searching only the requested
	directories (not their parents as well) and terminating the
	search loop as soon as the first occurrence of the requested
	object is found.

	(4) Searching the current directory can easily be achieved by
	including {} in TRAIL.SYS.

	(5) No extra code is required to handle {} occurring in
	TRAIL.SYS.

	(6) HOME is only searched if included in TRAIL.SYS (either as
	HOME or as { HOME }.

	(7) The code is independent of flag -55 (LAST ARGS).

	(8) The requested object is evaluated in the original
	directory, not the directory in which the requested object
	came from.  This is a personal preference.  I can provide a
	version which evaluates the object in it's own directory.
	Which option is preferred?  Should it be selectable via a
	flag?

Installation
============

Install EXEC in the HOME directory, make a list of directories to be
searched by EXEC and store in TRAIL in the HOME directory.