The /IF compiler directive is used to test a condition expression for conditional compilation.
/IF must be followed by at least one space, and then
the condition expression must be specified on the same line.
Following the condition expression, the remainder of the
line must be blank. 
In fixed-form, /IF must begin in column 7. 
/IF can be specified in any free-form statement
other than a free-form calculation statement.
Within free-form statements, /IF can begin in
column 8 or later.
See Conditional Directives Within a Free-Form Statement. 
If the condition expression is true, source lines following the /IF directive are selected to be read by the compiler. Otherwse, lines are excluded until the next /ELSEIF, /ELSE or /ENDIF in the same /IF group.