Defining substitution functions

[Timmy Burch]

I think is probably a painfully simple question, but I am really struggling 
to get started with Lilypond, so please take pity! I often have many measures 
with the same rhythm but different pitches, and I want to write a function 
that takes as arguments the pitches, and returns the formatted measure. An 
example of how I want things to work is given below. The two commented lines 
are how I want to invoke the function, and I want the output to be the two 
lines before the commented lines. This seems like it should be really easy to 
do, but I cannot make it happen. Thanks in advance for any help you can 
provide.

\version "2.16.2"

\score
{
    \context Staff
    {
        a4 c8 a8 c2 |
        b4 b8 b8 b2 |
        % \mymeasure a c
        % \mymeasure b b
    }
}