Avoiding Literal/Variable confusions

[wwitzel]

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One cannot distinguish between a Literal and its corresponding Variable form by looking at them and this can be confusing. We can take steps to avoid confusion, however.

1. Raise an exception when creating a Lambda expression that contains a Literal in its body whose Variable form is a parameter of a lambda. That is a -> f(_a) where '_a' represents the literal form of 'a' is not allowed. Alternatively, the conversion could be automatic, but raising an exception might be the safer option.
2. Implement the Expression diff feature of #278.
3. Anything else?