IF

        [SIC\]IF Logical_Expression [THEN]
        [SIC\]IF Logical_Expression Command [Arguments]

    The first form starts a conditional IF block. The behaviour of IF blocks
    is similar to structured Fortran. The THEN keyword is optional.
    The second form defines a logical IF statement, i.e. it  uses  a  single
    line  with  no possibility of alternate execution (no ELSE choice). When
    the argument following the logical expression is not THEN, it is assumed
    to  be a command which will be interpreted, together with the next argu-
    ments, if the logical expression evaluates as true.

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

    The  logical expression must be a single argument (no blanks); composite
    expressions using .AND. or .OR. operators are supported.  Logical  func-
    tions  like EXIST, FILE, or FUNCTION can also be used (see HELP FUNCTION
    for details).