Expression Engine Handles Falsies Improperly

[Enthalpy]

The expression engine will return true for an expression using variables defined as 0, but false for an expression using undefined variables. Ideal behavior would have both return false, using the idea of "falsies." A fix would risk breaking things for trials that depend on whether a variable has been defined or not. However, that would be a rather fragile hack that should be fixable on the author's end with minimal additional effort.

Consider this when fixing Issue #101. The whole scope of the truthy/falsy problem also should be determined.