Failure to account for dependencies when checking for creation of a new variable

[WilCrofter]

In several places we test if an expression creates a new variable by evaluating the expression in a clean environment. This would fail if the "right side" of an assignment depended upon previously defined variables which are inaccessible from the clean environment.

By luck, this does not seem to affect current content.

[WilCrofter]

Closed as included in #39