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).