# operators: the docstrings contain the
# symbol associated to each operator
def op_add(x, y):
    '+'
    return x+y

def op_sub(x, y):
    '-'
    return x-y

# entry-point
def main(argv):
    try:
        print calculate(argv[1:])
        return 0
    except SyntaxError:
        print 'Invalid expression'
        return 1

# evaluate a list of string
def calculate(lst):
    stack = []
    for token in lst:
        try:
            val = int(token)
        except ValueError:
            op = OPCODES.get(token, None)
            if op is None:
                raise SyntaxError
            y, x = stack.pop(), stack.pop()
            val = op(x, y)
        stack.append(val)
    if len(stack) != 1:
        raise SyntaxError
    return stack[0]

# INIT-TIME only: automatically build the
# table of opcodes by inspecting all the 
# functions declared and their docstrings
OPCODES = {}
def build_opcodes():
    'NOT_RPYTHON'
    for name, value in globals().items():
        if name.startswith('op_'):
            OPCODES[value.__doc__] = value
build_opcodes()