An interface block can be used to specify a generic name to
  reference all of the procedures within the interface block.
  Statement format for initial line in block:

     INTERFACE generic-name

  This kind of interface block can be used to extend or redefine a
  generic intrinsic procedure.

  The procedures that are given the generic name must be the same
  kind of subprogram:  all must be functions, or all must be
  subroutines.

  Any procedure reference involving a generic procedure name must be
  resolvable to one specific procedure; it must be unambiguous.

  EXAMPLES:

    INTERFACE GROUP_SUBS
      SUBROUTINE INTEGER_SUB (A, B)
        INTEGER, INTENT(INOUT) :: A, B
      END SUBROUTINE INTEGER_SUB

      SUBROUTINE REAL_SUB (A, B)
        REAL, INTENT(INOUT) :: A, B
      END SUBROUTINE REAL_SUB

      SUBROUTINE COMPLEX_SUB (A, B)
        COMPLEX, INTENT(INOUT) :: A, B
      END SUBROUTINE COMPLEX_SUB
    END INTERFACE

  The three subroutines can be referenced by their individual
  specific names or by the group name GROUP_SUBS.

  The following example shows a reference to INTEGER_SUB:

  INTEGER V1, V2
  CALL GROUP_SUBS (V1, V2)