Creating functions in snippets, and how to call them?

// How do I call snippet functions from my pages, or even within snippets?
function pickDate($filter_date)
function pickMonth($filter_month)
function pickYear($filter_year)

global $modx

function myFunc($args) {
global $modx;
// function code
}

// On some page, or another chunk
// the name of the snippet is datestuff
// but how do I call a function from this point?
[[datestuff]]

[!snippet? &function=`daestuff`!]