Use the /UNDEFINE directive to indicate that a condition is no longer defined.
/UNDEFINE must be followed by at least one space, and then
the condition-name must be specified on the same line.
The remainder of the line must be blank.
See Compiler Directives for information on
the columns available for directives.
/UNDEFINE cannot be specified within
a free-form statement.
The /UNDEFINE directive removes a condition-name from the list of currently
defined conditions. A subsequent /IF DEFINED(condtion-name) would be false.
A subsequent /IF NOT DEFINED(condition-name) would be true.
Note: Any conditions specified on the DEFINE parameter will be considered to be
defined when processing /IF and /ELSEIF directives. These conditions can be
removed using the /UNDEFINE directive.