dist/picturefill.js
function(fn) {
		var cache = {};
		return function(input) {
			if ( !(input in cache) ) {
				cache[ input ] = fn(input);
			}
			return cache[ input ];
		};
	}
function(fn) {
		var cache = {};
		return function(input) {
			if ( !(input in cache) ) {
				cache[ input ] = fn(input);
			}
			return cache[ input ];
		};
	}
simple memoize function: