Support for not() builtin

[myitcv]

Noting an offline conversation with @mpvl and @rogpeppe

Is your feature request related to a problem? Please describe.

Consider:

Steps: [name=string]: #Step | 
    { [#LangCode]: #Step } | 
    { [#ScenarioName]: #Step } | 
    { [#LangCode]: [#ScenarioName]: #Step } | 
    { [#ScenarioName]: [#LangCode]: #Step }

#LangCode = "en" | "fr" // etc

Scenarios: [name=XYZ]: #Scenario

#ScenarioName: or([ for name, _ in Scenarios {name}])

The declaration of Steps as written is only possible if we can constrain a #Scenario name in some way (shown with XYZ here) to be any string that is not a valid #LangCode.

Describe the solution you'd like

Potentially something like:

not(or(#LangCode))

Describe alternatives you've considered

A negative specification using _|_ does work:

Scenarios: [#LangCode]: _|_

However, it leads to more misleading error messages.

Additional context

[myitcv]

Noting an offline conversation: that not() is effectively syntactic sugar around constrain():

x: not(y)

// is equivalent to

x: constrain(x & y == _|_)

Which might mean that not() is not required (assuming "nice" error messages can be inferred from such negative constraints).

[cueckoo]

This issue has been migrated to cue-lang/cue#571.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.