turns out to be somewhat tricky.
The easy shot would be to use the comma operator,
but I don't like that idea, since in logic programming, comma means "and then".
So I prefer an || operator, similar to short-circuit evaluation of boolean OR
Unfortunately, OR binds stronger than assignment, so we need to trick our way
into a smooth DSL syntax by wrapping into intermediary marker types, and accept
rvalue references only, as additional safeguard to enforce the intended inline
definition syntax typical for DSL usage.
