Assignments native

At the end of Sec 6 of the 2006 Modest paper some abbreviations are defined.

It seems (at least) this one is not supported in the current version.

In a similar line, conventional like y := 3; are to be read as {= y:=3 =};

It would be good to have that built in. The intention is that standard sequential programming (including also while/for etc) have native support in Modest (albeit at the price of inducing many internal transitions, which can be optimised away by good static analysis/compilation).