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. 
In fixed-form, /UNDEFINE must begin in column 7. 
/UNDEFINE cannot be specified within
a free-form statement.
However, between free-form statements, /UNDEFINE can begin in
column 7 or later. 