Executes a block of statements if the logical expression is true.
  The block of statements starts immediately following the IF
  statement.  The block of statements can be followed by optional
  ELSE IF statements (any number) and one optional ELSE statement.
  The entire block IF construct must be terminated by an END IF
  statement.  Format:

     [name:] IF (e) THEN
       block
     [ELSE IF (e1) THEN [name]
       block]...
     [ELSE [name]
       block]
     END IF [name]

     name   Is the name of the IF construct.

     e,e1   Are logical expressions.

     block  Is a series of zero or more Fortran statements
            (called a statement block).

  If a construct name is specified in a block IF statement, the same
  name must appear in the terminal END IF statement.  If no construct
  name is specified in the block IF statement, no name can appear in
  the terminal END IF statement.  The construct name must be a unique
  identifier in the program unit.

                                 NOTE

          No additional statement can be placed after the  IF
          THEN  statement  in  a  block  IF  construct.   For
          example, the following statement is invalid in  the
          block IF construct:

             IF (e) THEN I = J

          This  statement  is  translated  as  the  following
          logical IF statement:

             IF (e) THENI = J