This doesn’t work for me:

return Multiply(Sum(a1, a2), RiseFall(dur, 0.5))

Thankfully, Python allows us to overload the operators, so we can express the same line as this:

return (a1 + a2) * RiseFall(dur, 0.5)

Less typing, easier to read. Let’s see it in the context of @Instr MyInstr:

@Instr
def MyInstr(dur=1.0, amp=1.0, freq=440, tune=12):
    a1 = Sine(amp * 0.5, freq)
    a2 = Sine(amp * 0.5, freq * 2 ** (tune / 12.0))
    return (a1 + a2) * RiseFall(dur, 0.5)

Here’s how I implemented it. I started by creating a generic iterator class called UnitGenerator:

class UnitGenerator:
    def __init__(self): pass
    def __iter__(self): pass                 
    def next(self): raise StopIteration    
    def __add__(self, i): return Add(self, i)
    def __mul__(self, i): return Mul(self, i)

The last two lines of the class redefine __add__() and __mul__(), which control the behaviors of + and *. These functions use the custom classes Add() and Mul(). These were originally called Sum() and Multiply(), though I renamed them to follow Python naming conventions. The last thing I had to do was alter some of the existing classes to derive from class UnitGenerator, so they automatically incorporate the overloaded operators.

class Instr(UnitGenerator):  ...
class IterReduce(UnitGenerator):  ...
class Mul(IterReduce):  ...
class Add(IterReduce):  ...
class RiseFall(UnitGenerator):  ...
class Sine(UnitGenerator):  ...

Classes Mul and Add are also of type UnitGenerator. They inherit for class IterReduce which inherents from UnitGenerator.