undef Statement--Undefine a Substitution Variable
Use the ##undef statement to undefine a substitution variable defined with the ##define statement. You can only undefine a user-defined substitution variable.
This statement has the following syntax:
##undef $variable
Example--undef statement:
The following statements indicate the value of the variable $status, and undefine it, if necessary.
##ifdef $status
/* '$status' is defined as $status */
##undef $status
/* '$status' is now undefined */
##else
/* '$status' is undefined */
##endif