Builtin Helpers

---

Several new builtin functions are proposed to address issues and inconsistencies.

Replacing _|_ (bottom):

• error to enable user defined messages
• isconcrete and exists for testing or comparisons

Validators:

• must(expr) passes if expr evaluates to true and fails otherwise
• not(expr) passes if unified with a value x for which expr&x fails and false otherwise.
• numexist(count, ...expr) passes if the number of expressions for which exists(x) evaluates to true unifies with count.
• numconcrete(count, ...expr) passes if the number of expressions for which isconcrete(x) evaluates to true unifies with count.
• numvalid(count, ...expr) passes if the number of expressions for which isvalid(x) evaluates to true unifies with count.

Others:

• manifest(x) evaluates x stripping it of any optional fields and definitions and disambiguating disjunctions after their removal.
• range(from, to, by) creates a stream of values like found in other languages
• head([...]) returns the first element of a list
• Package std would namespace all core builtins under the std package name.

You can find more details on GitHub issue: core builtin extensions