Non-empty list of dummy variables

In the documentation, it is written, that the list of dummy variables of an internal function may not be empty. This is not true, for example the input f() = 1; print(f()) is parsed and evaluated without an error. In addition, there are two or three tests using a function with no arguments. The docs and the implementation should be made consistent. As it is not very likely to have an use case requiring a function with no dummies, I would suggest to stop supporting it, i.e. to implement it as it is in the docs. For similar reasons, series literals may not be empty. Empty series as result of operations may be empty but a function definition cannot be a result of operations in this language, i.e. functions cannot be returned. Therefore, there is no need for functions with empty list of dummies.