Docstrings for classes

diff --git a/data.py b/data.py
index 5008fdd..96f5c63 100644
--- a/data.py
+++ b/data.py
@@ -29,11 +29,13 @@ def is_nothing(x):
     return isinstance(x,NothingClass)

 def obj(x):
+    "Convert x to object - Const or anything else"
     if isinstance(x,float) or isinstance(x,int):
         return Const(x)
     return x

 def sqrt_(x):
+    "Square root for any type of object"
     if is_number(x):
         if x < 0:
             return Nothing
@@ -47,6 +49,7 @@ def sqrt_(x):
         return Expr('sqrt',x,None)

 class NothingClass(object):
+    "This class represents `no result' value"
     def __repr__(self):
         return "Nothing"

@@ -71,6 +74,7 @@ class NothingClass(object):
 Nothing = NothingClass()

 class Unknown(object):
+    "Unknown variable"
     def __init__(self,name='',add=True,value=None):
         self.name = name
         if add:
@@ -103,6 +107,7 @@ class Unknown(object):
         return Expr('/',self,e)

 class Const(float):
+    "Numeric constant"
     def __init__(self,x):
         float.__init__(self,x)

@@ -142,6 +147,7 @@ class Const(float):
         return Expr('*',self,e)

 class Line(object):
+    "Straight line"
     def __init__(self, A,B,C):
         self.A = A
         self.B = B
@@ -195,6 +201,7 @@ def segment(name):
         return p

 class Segment(object):
+    "Segment of a straightline - class for syntax parser"
     def __init__(self,A,B):
         self.A = A
         self.B = B
@@ -203,6 +210,7 @@ class Segment(object):
         return "[%s--%s]" % (self.A, self.B)

 class Sum(list):
+    "This class represents polynomials"
     def __repr__(self):
         return " + ".join(map(repr,self))

@@ -221,6 +229,7 @@ class Sum(list):
             return Sum(lst)

 class UD(object):
+    "Dictionary, which shows power for each unknown in Monoid"
     def __init__(self,dct):
         self.dict = {}
         for k,v in dct.iteritems():
@@ -314,6 +323,7 @@ class Monoid(object):
             return True

 class Expr(object):
+    "Generic expression class"
     def __init__(self,op,x,y):
         self.op = op
         self.x = x
@@ -354,7 +364,7 @@ class Expr(object):
             elif self.op == '+':
                 return Expr('+', self.x*e, self.y*e)
         return Expr('*',self,e)
-
+
     def __div__(self,e):
         if is_number(e):
             if self.op == '*':
@@ -367,6 +377,7 @@ class Expr(object):
         return Expr('/',self,e)

 def to_sum(expr):
+    "Convert anything to Sum"
     if isinstance(expr,Monoid):
         return Sum([expr])
     if isinstance(expr,Sum):
@@ -384,6 +395,7 @@ def to_sum(expr):
         return to_sum(expr.x) * to_sum(expr.y)

 class Union(set):
+    "This class represents `many results' values"
     def __repr__(self):
         lst = list(self)
         return repr(lst)
@@ -428,7 +440,7 @@ class Union(set):
         for x in self:
             n.add(x**e)
         return n
-
+
     def __div__(self,e):
         if isinstance(e,Union):
             n = Union()
@@ -442,8 +454,6 @@ class Union(set):
                 n.add(x/e)
             return n

-
-
 def collect_nums(sum):
     s = 0
     r = Sum([])