#!/usr/bin/env python2.7

def query(c):
    return sum(c.itervalues())

def merge(c1, c2):
    return {k: max(c1.get(k, 0), c2.get(k, 0)) for k in set(c1).union(c2)}

class tx(object):
    def __init__(self, id):
        self.id = id

    def add(self, c, n=1):
        assert n >= 0
        c = dict(c)
        c.setdefault(self.id, 0)
        c[self.id] += n
        return c

c1 = dict(tx1=7)
print "c1", query(c1)

c2 = tx(2).add(c1)
print "c2", query(c2)

c3 = tx(3).add(c1)
print "c3", query(c3)

c4 = merge(c2, c3)
print "c4", query(c4)