Syntax: DEFINE name value
or      DEFINE name { value }
or      DEFINE name < value >
or      DEFINE name DELETE
or      DEFINE name LOCAL
or      DEFINE name ( expr )
or      DEFINE name :
or      DEFINE name ? [ < prompt > ]
or      DEFINE name |
or      DEFINE name READ INTEGER
or      DEFINE name READ INTEGER1 INTEGER2
or      DEFINE name IMAGE
or      LIST DEFINE [start end]

   All these commands define variables, which may be accessed by
$name, in which case $name is replaced by its value.  Variables are
not expanded within {...} or "...", if you want to force expansion
within double quotes, use the form $!name, $!!name is expanded
anywhere.  Name is any string starting with a letter or _, and
containing only letters, digits or _.  Keywords are valid variable
names. If you only want to know if a variable is defined, then $?name
has value 1 if it is, 0 otherwise.  (Note that some variables such as
`date' or `angle' are special as they always contain an up-to-date
value, for an example `echo $date' sometime).

   DEFINE name value, DEFINE name { value }, and DEFINE name < value >
define name to be value, which may be a word, a number, or a list.
DEFINE name DELETE deletes a variable.  DEFINE name ( expr ) sets
variable name to be the value of a (scalar) expression; see ARITHMETIC
and LOGICAL. The expression is permitted to be an element of a string
vector (of course, arithmetic vectors are also allowed).

   DEFINE name LOCAL makes name local, that is, it is visible only in the
current macro and macros called from it. If a variable of the same name
already exists it will be invisible until you leave the macro containing
the LOCAL command. You cannot set variables to be LOCAL except in macros.

   DEFINE name : reads the value of name from the environment file.
If it can't be found, and is capitalised, SM will look for it in the
environment (as a logical variable for VMS).  DEFINE name ? will
prompt you for the value of `name', using your prompt if specified.

   DEFINE name | is used to define a variable from an internal SM
variable such as expand or angle.  The variables accessible are listed
in the manual under DEFINE, or may be listed with LIST DEFINE |, and many
of them are used in the macro `show'.

   These variables are all defined for you when SM starts and each is
always correct, tracking the current value of the corresponding
internal variable.  For example, try "echo $angle\n angle 45 echo
$angle". If you now say define angle |, $angle will cease to track the
internal value and will remain fixed (the same effect can be achieved
with define angle 45). When you say define angle delete it will once
more track the internal value. Your old code will continue to work,
but in many cases it is possible to remove the explicit definition
with |. This special sort of variable will not be SAVEd, and will not
show up if you list the currently defined variables.

   DEFINE name READ INTEGER sets name to be line INTEGER of the
current data file, DEFINE name READ INTEGER1 INTEGER2 defines name to
be word INTEGER2 of line INTEGER1.  Name is subject to the same
restrictions as any variable.

   DEFINE name IMAGE defines a variable from a file read with the
IMAGE or TABLE command. Currently this only works for NX, NY, X0, X1,
Y0, Y1, or any keyword from a FITS header.

   LIST DEFINE lists all the current variables (or use the macro `lsv')