def compile(program):
    memo = []
    for ast in program:
        memo += compile_one(ast)
    return memo

def compile_one(ast):
    inst = ast[0]
    if inst == 'assign':
        lhs = ast[1]
        rhs = ast[2]
        x = compile_one(rhs)
        return x + [('set', lhs)]
    elif inst == 'refer':
        return [('get', ast[1])]
    elif inst == 'value':
        return [('put', ast[1])]
    elif inst == 'plus':
        arg1 = ast[1]
        arg2 = ast[2]
        return compile_one(arg1) + compile_one(arg2) + [('plus',)]
    elif inst == 'print':
        name = ast[1]
        return compile_one(name) + [('print',)]
    elif inst == 'variables': #
        return [('variables',)]
    else:
        return 'omg: ' + inst


def execute(bytecode):
    stack = []
    variables = {}
    for nme in bytecode:
        inst = nme[0]
        if inst == 'put':
            stack.append(nme[1])
        elif inst == 'set':
            name = nme[1]
            variables[name] = stack.pop()
        elif inst == 'get':
            name = nme[1]
            stack.append(variables[name])
        elif inst == 'plus':
            a = stack.pop()
            b = stack.pop()
            stack.append(a + b)
        elif inst == 'print':
            a = stack.pop()
            print a
        elif inst == 'variables':
            print variables

# a = 1
# b = a + 2
# print b
program = [
        ('assign', 'a', ('value', 1)),
        ('assign', 'b', ('plus', ('refer', 'a'), ('value', 2))),
        ('print', ('refer', 'b')),
        ('variables',)]
# put 1
# set a
# get a
# put 2
# plus
# set b
# get b
# print

bytecode = compile(program)
execute(bytecode)