add_point_to_trajectory Subroutine

    subroutine add_point_to_trajectory(me,et,x)

    implicit none

    class(trajectory),intent(inout) :: me !! the trajectory
    real(wp),intent(in) :: et
    real(wp),dimension(6),intent(in) :: x

    if (.not. allocated(me%et)) then
        allocate(me%et(1)); me%et(1) = et
        allocate(me%x(1));  me%x(1)  = x(1)
        allocate(me%y(1));  me%y(1)  = x(2)
        allocate(me%z(1));  me%z(1)  = x(3)
        allocate(me%vx(1)); me%vx(1) = x(4)
        allocate(me%vy(1)); me%vy(1) = x(5)
        allocate(me%vz(1)); me%vz(1) = x(6)
    else
        me%et = [me%et, et   ] ! auto lhs allocation
        me%x  = [me%x,  x(1) ]
        me%y  = [me%y,  x(2) ]
        me%z  = [me%z,  x(3) ]
        me%vx = [me%vx, x(4) ]
        me%vy = [me%vy, x(5) ]
        me%vz = [me%vz, x(6) ]
    end if

    end subroutine add_point_to_trajectory