I guess one interpretation of Erlang's = (this is from someone rather new to the language) is "Make the thing on the left equal to the thing on the right withou

I guess one interpretation of Erlang's = (this is from someone rather new to the language) is "Make the thing on the left equal to the thing on the right without mutating existing variables. If that's impossible, error". That nicely accounts for the apparent dual nature of the = operator.