gistfile1.py

class Var:
  def __init__(self, name):
    self.name = name
    
  def execute(self, bindings):
    return bindings[self.name]


class ConstInt:
  def __init__(self, val):
    self.val = val
    
  def execute(self, bindings):
    return self.val


class Add:
  def __init__(self, left, right):
    self.left = left
    self.right = right
    
  def execute(self, bindings):
    return self.left.execute(bindings) + self.right.execute(bindings)

    
class Mul:
  def __init__(self, left, right):
    self.left = left
    self.right = right
    
  def execute(self, bindings):
    return self.left.execute(bindings) * self.right.execute(bindings)