Who are the muppets in Greg Smith's Goldman Sachs letter?

From guardian.co.uk

Resigning banker Greg Smith claimed clients were referred to as 'muppets' to be ripped off. Time for some muppetology. Two weeks ago today, former Goldman Sachs employee Greg Smith published a scathing open letter in the New York Times. The "executive director and head of equity derivatives sales" lashed out at senior leadership for creating what he claimed was a "toxic" and "destructive" culture that put the interests of the investment bank ahead of those of its clients – who he said were routinely referred to in emails as "muppets" to be "ripped ... (full story)