double purity(String codon, String around) {
		String pre = around.substring(0, CODONS_SIZE * CODONS_AROUND);
		String post = around.substring(around.length() - CODONS_SIZE * CODONS_AROUND);
		if( debug ) Gpr.debug("codon: '" + codon + "'\tAround: '" + around + "'\tpre: '" + pre + "'\tpost: '" + post + "'");

		// Count all codons
		HashMap<String, Integer> count = new HashMap<String, Integer>();

		// First use 'codons'
		int maxSyms = codon.length() / CODONS_SIZE;
		for( int i = 0; i < maxSyms; i++ ) {
			String sym = getCodon(codon, i);
			addAa(count, sym, 1);
		}

		// Now count codons 'around' (pre and post)
		for( int i = 0; i < CODONS_AROUND; i++ ) {
			String cod = getCodon(pre, i);
			addAa(count, cod, 1);

			cod = getCodon(post, i);
			addAa(count, cod, 1);
		}

		// Find AA-codon purity 
		return maxPurity(count);
	}