notehead overlap

[pabuhr]

Is there an easy way to get the 1st/9th noteheads to overlap like the 7th/15th
noteheads overlap?

\version "2.16.0"
\language english
#(set-global-staff-size 25)

su = \stemUp
sd = \stemDown

melody = \relative c'' {
        << { \sd a,4. a8 } \\ { \su a16 a' c a c a a, a'} >> << { \sd d,4. d8 } 
\\  { \su d16 a' b a b a d, a' } >> |
} % melody

\score {
        <<
                \context Voice {
                        \key c \major \time 4/4
                        \melody
                }
        >>
} % score