No way to express a relation with zero attributes?

[dportas1]

I can't create a relation with zero attributes or return zero attributes from an expression (i.e the relations DUM and DEE).

E.g. the following give syntax errors

π (R)
A = {}

[rlaiola]

Here comes a gift 🎁 in the 6 years anniversary of this request 😃

#212

Test it live at https://rlaiola.github.io/relax/calc/local/uibk/local/0.

Dum and Dee relations can be represented as follows:

Dum = {}   -- inline relation with empty heading and no tuples
Dee = {()} -- inline relation with empty heading but one tuple of degree zero

Given a relation R with n (n > 0) attributes and t (t > 0) tuples

R x Dum -- returns a relation with the same schema of R but no tuples
R x Dee -- returns the very same relation R