IF

        [SIC\]IF Logical_Expression [THEN]

    Starts a conditional IF block. The behaviour of IF blocks is similar  to
    structured  FORTRAN.  Any  text  following  the  logical  expression  is
    ignored, but if this text is not THEN, an error message is produced.

    This command can only be used within a procedure: interactive users  are
    expected to be able to make their decisions themselves...

    Within logical expressions, two special logical functions can be used:
         EXIST(Name)    Returns .TRUE. if Name is an existing variable
         FILE(Name)     Returns .TRUE. if Name is an existing file
    and composite expressions are allowed.