go Subroutine

    subroutine go(puzzle, ileft, iskip)
    !! compute the number of cols to left of mirror (0 if none)
    !! call with transpose(puzzle) to get number of rows above.
    integer,dimension(:,:),intent(in) :: puzzle
    integer,intent(out) :: ileft
    integer,intent(in),optional :: iskip !! don't consider this column

    integer :: nrows, ncols, i, j
    logical :: mirror

    nrows = size(puzzle,1)
    ncols = size(puzzle,2)
    ileft = 0
    do i = 1, ncols-1
        if (present(iskip)) then
            if (i==iskip) cycle
        end if
        mirror = .true.
        do j = 1, min(i, ncols-i)
            if (any( puzzle(:,i-j+1) /= puzzle(:,i+j)) ) then
                mirror = .false.  ! no mirror on this line
                exit
            end if
        end do
        if (mirror) then
            ileft = i
            exit
        end if
    end do
    end subroutine go