Why is this pure function foreign?

[ursi]

trypurescript/staging/src/TryPureScript.js

Lines 9 to 15 in 0f64855

	exports.encode = function(text) {
	return text
	.replace('<', '&lt;')
	.replace('>', '&gt;')
	.replace('&', '&amp;')
	.replace('"', '&quot;');
	};