Conditionally transfers control to one of three statements, based
 on the current value of an arithmetic expression.  Statement
 format:

    IF (e) s1,s2,s3

    e         Is an arithmetic expression.

    s1,s2,s3  Are labels of executable statements in the same 
              program unit.  All three labels are required, 
              but they need not refer to different statements.

 Executes the statement at the first label ("s1") if the arithmetic
 expression evaluates to a value less than 0; the statement at the
 second label ("s2") if the arithmetic expression evaluates to 0; or
 the statement at the third label ("s3") if the arithmetic
 expression evaluates to a value greater than 0.

 Executes the statement if the logical expression is true.
 Statement format:

    IF (e) st

    e   Is a logical expression.

    st  Is a complete Fortran statement. The statement can 
        be any statement except DO, END DO, END, block IF, 
        or another logical IF statement.

 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:

    IF (e) THEN
      block
    ELSE IF (e1) THEN
      block
    ELSE
      block
    END IF

    e,e1   Are logical expressions.

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

 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