Regular formulas cannot contain the `nil' element.

[jjmartens]

According the book Appendix C.3.2 and the website the regular formula for the empty sequence (epsilon) has plain notation nil.

However, it seems that this is not implemented. For now we have to use false*, which seems not very descriptive.

[jjmartens]

Historic pointers that might be relevant:

• Formulas of the shape <nil>\phi and [nil]\phi lead to crash. #843
• 7432cda
• d052fbb

[mlaveaux]

In the mCRL2 meeting it was discussed that we should add 'nil' to the toolset, and that it was unclear why it was previously removed.