diff --git a/src/gf256.py b/src/gf256.py
--- a/src/gf256.py
+++ b/src/gf256.py
@@ -1,4 +1,8 @@
+"""Arithmetic operations on Galois Field 2**8."""
+
+
 def _ffmul(a, b):
+	"""Basic multiplication."""
 	r=0
 	while a!=0:
 		if (a&1)!=0: r^=b
@@ -9,19 +13,20 @@ def _ffmul(a, b):
 	return r
 
 
-g=3
-E=[None]*256
-L=[None]*256
+g=3 # generator
+E=[None]*256 # exponentials
+L=[None]*256 # logarithms
 acc=1
 for i in range(256):
 	E[i]=acc
 	L[acc]=i
 	acc=_ffmul(acc, g)
 L[1]=0
-inv=[E[255-L[i]] if i!=0 else None for i in range(256)]
+inv=[E[255-L[i]] if i!=0 else None for i in range(256)] # multiplicative inverse
 
 
 def ffmul(a, b):
+	"""Fast multiplication. Basic multiplication is expensive. a*b==g**(log(a)+log(b))"""
 	if a==0 or b==0: return 0
 	t=L[a]+L[b]
 	if t>255: t-=255
@@ -29,6 +34,9 @@ def ffmul(a, b):
 
 
 def evaluate(coefs,x):
+	"""Evaluate polynomial's value at x.
+
+	:param coefs: [a0, a1, ...]."""
 	res=0
 	xK=1
 	for a in coefs:
@@ -37,7 +45,9 @@ def evaluate(coefs,x):
 	return res
 
 
-def getConstantCoef(k,*points):
+def getConstantCoef(*points):
+	"""Compute constant polynomial coefficient given the points."""
+	k=len(points)
 	res=0
 	for i in range(k):
 		(x,y)=points[i]