Programming Sine Waves in Scheme
    A. Summing Two Sine Waves:
        1. to get beating behavior:     listen     see
        2. to get a fused, complex timbre:     listen     see
        3. to get a multi-pitch timbre:     listen     see
    B. Amplitude Modulation (AM):
        1. to get beating behavior:     listen     see
        2. to get a fused, complex timbre:     listen     see
        3. to get a multi-pitch timbre:     listen     see
    C. Frequency Modulation (FM):
        1. to get beating behavior:     listen     see
        2. to get a fused, complex timbre:     listen     see
        3. to get a multi-pitch timbre:     listen     see