Section 13,38: IMPLICIT_NONE

 Disables the implicit declaration of data types in the program
 unit.  When it is used, you must declare the data types of all
 symbols explicitly.  You must not include any other IMPLICIT
 statements in the program unit containing an IMPLICIT NONE
 statement.  Statement format:

    IMPLICIT NONE

 NOTE:  To receive warnings when variables are used but not
 declared, you can specify the -warn declarations compiler option
 instead of IMPLICIT NONE.