SUBMODULE error #6645: The name of the module procedure conflicts....

Loaded PSXE2016 start night and today had a first play with the much awaited SUBMODULES. consider the following code:

MODULE BILL
    implicit none
    public
    integer, parameter          :: axs6_irk = kind(1.D0)            ! real kind for dp
    integer, parameter, private :: irk = axs6_irk
    INTERFACE
        MODULE FUNCTION IS_LEAP_YEAR(IYEAR)
            INTEGER(KIND=2), INTENT(IN) :: IYEAR
            LOGICAL(KIND=4) :: IS_LEAP_YEAR
        END FUNCTION IS_LEAP_YEAR
        MODULE SUBROUTINE DVECRS(V1XV2,V1,V2)
            REAL(KIND=IRK) :: V1XV2(3)
            REAL(KIND=IRK), INTENT(IN) :: V1(3)
            REAL(KIND=IRK), INTENT(IN) :: V2(3)
        END SUBROUTINE DVECRS
    END INTERFACE
END MODULE BILL

This compiles OK with no errors or warnings. Then we have.

submodule (BILL) BILL_SUBS1
    contains
    function is_leap_year(iyear)
       ! returns true if iyear is a leap year and false otherwise
       implicit none
       logical(4)              :: is_leap_year
       integer(2), intent (in) :: iyear
       if     (mod(iyear,400_2) == 0) then ! special case for every 4th century
           is_leap_year=.true.
       elseif (mod(iyear,100_2) == 0) then ! centuries are not leap years
           is_leap_year=.false.
       elseif (mod(iyear,4_2) /= 0 ) then  ! not divisible by 4 and not special case so not a leap year
           is_leap_year=.false.
       else
           is_leap_year=.true.             ! divisible by 4 and not a special case to leap year
       endif
    end function is_leap_year
    subroutine dvecrs(V1XV2,V1,V2 )
        !     THIS FORMS THE CROSS PRODUCT OF V1 AND V2 AS V1XV2
        implicit none
        REAL(irk)             :: V1XV2(3)
        REAL(irk), intent(in) :: V1(3) , V2(3)
        V1XV2 = [V1(2)*V2(3) - V1(3)*V2(2), -V1(1)*V2(3) + V1(3)*V2(1), V1(1)*V2(2) - V1(2)*V2(1)]
    end subroutine dvecrs
end submodule BILL_SUBS1

This was a simple example using one function and one subroutine. I get the errors>>>>

sample2.f90(3): error #6645: The name of the module procedure conflicts with a name in the encompassing scoping unit.   [IS_LEAP_YEAR]

sample2.f90(18): error #6645: The name of the module procedure conflicts with a name in the encompassing scoping unit.   [DVECRS]

what have a done wrong? I have read various example etc and I can't see it though I may be suffering that brand of selective blindness that sometime filters out the glaring errors...