1INCLAN: eval, show, set, unset, external - set, show and unset variables

Usage: eval <variable> = <expression>
       show <variable> ...
       set <variable> ...
       set <variable> = <value>
       unset <variable> ...
       <variable> = <expression>
       <variable> := <expression>
       external <variable> = <expression>
       external <variable> := <expression>

The "eval" statement evaluates the Fortran-77 <expression> and assigns the
result to the <variable>.

The statement "show" without parameters lists the names and values of all 
global variables that do not have the value "NULL"; with parameters, "show"
lists the names and values of the given variables. Global variables that are
hidden by a local variable with the same name are identified by the attribute
"hidden".

The statement "set" without parameters lists the names and values of all
variables that do not have the value "NULL"; with parameters, "set" lists
the names and values of the given variables. In contrast to "show", local
variables are listed too.

"set <variable> = <value>" gives the variable <variable> the string value 
<value>.

The statement "unset" the given variables. System variables cannot be unset.

"<variable> = <expression>" and "<variable> := <expression>" are shorthand
notations for "eval <variable> = <expression>" and "set <variable> =
<expression>", respectively.

"external <variable> := <value>" or "external <variable> = <value>" assign
a value (i. e. a string) or the result of an expression to an external 
(non-local) variable even if a local variable with the same name exists. 
This command can be used to return values from a macro to the calling macro.