Expressions not having enough operands are not covered by the test data.

[stof]

While investigating the code coverage of my PHP implementation, I found out one error case that was never triggered by the testsuite.
It corresponds to the case where an operand is missing after an operator:

• a and
• a or
• not
• a and not

The existing error text I have in PHP is Tag expression "a and" could not be parsed because of an empty stack., which is a message copied from the Java implementation I used as reference.
The JS implementation seems to use empty stack as message.
Ruby seems to use Empty stack as message
Python seems to provide a more meaningful message saying Too few operands (expressions=%r)
No idea about the error message of Perl.

In any case, this "empty stack" mention is not a user-friendly error message.

[mpkorstanje]

Feel free to create a pull request to add some cases to errors.yml and we can tackle that.

[stof]

I might do it, but I'd rather wait until #194 is merged, so that I can also work on the PHP implementation.