Executes the statement if the logical expression is true.  In
  Fortran 95/90, this is called an IF statement (as compared to block
  IFs, which are called IF constructs).  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,
         CASE, FORALL, or WHERE constructs, or another logical
         IF statement.