Wrote some more comments about the code.

expression.py

@@ -1,8 +1,10 @@
 class Expression:
-	expression = ""
+	# This object represents all the needed initial data about an expression that is to be parsed.
+	
+	expression = "" # The formula to parse as a string.
 #	correct_response = 0
-	wrong_response = 0
-	variables = []
+	wrong_response = 0 # The incorrect response that was entered by the student.
+	variables = [] # A list of Variable.
 	
 	def __init__(self, expression_string, variables, wrong_response):
 		self.expression = expression_string
@@ -10,10 +12,13 @@ class Expression:
 		self.wrong_response = wrong_response
 		
 class Variable:
+	# This object represents a variable that is found in the formula. It encodes the name, the type of variable
+	# and the starting point from which to look for mutations in the variable's value.
+	
 	name = ""
 	type = ""
 	starting_point = 0
-	ending_point = 0
+#	ending_point = 0
 	
 	VARIABLE_TYPES = {
 		"i" : "Interest",
@@ -32,6 +37,9 @@ class Variable:
 		
 	@staticmethod
 	def get_variable_type(variable_name):
+		# Utilitie function to get the variable's type from it's name.
+		# For example, a variable with 'i' in its name is an Interest type.
+		# Returns the variable's type as a string or 'Unknown' otherwise.
 		if not variable_name in Variable.VARIABLE_TYPES:
 			return "Unknown"
 		return Variable.VARIABLE_TYPES[variable_name]