should be wrapped to form an expression
More informative error message for unbound variable
Introduce the alias `Sig` (short for signature) for `list[Def]`
The module AST instead of Syntax should be imported for the evaluator
To support simple variable mutation, an environment monad is sufficient
Finish the evaluator, not tested yet
Add tests for the evaluator
Fix tons of bugs in the evaluator; now it passes all tests in Tests.rsc
Merge branch 'master' of github.com:seba--/hygienic-transformations