performance.pl: Divide long usage example into several lines
Document make_instances/3 and make_instance/3
Automatically update Menu.txt by NaturalDocs
Clarify documentation of make_instance/3
Fix type declaration of make_instance/3
Add example of usage in documentation of make_instance/3
Remove unused predicate members/2 from srp.pl
Reimplement elements/3 using maplist/4
Add nicer documentation to elements/3
Add tests to elements/3
Consult 'random.pl' in 'preferences.pl' so that range/2 is available
Consult 'srp.pl' and 'random.pl' in 'performance.pl'

This is to make srp/3 and random_preferences/2 available.
Add 'valid_preferences.pl' with esp. valid_preferences/1

This new predicate can be used for validating SRP instances.
Rename file 'random.pl' -> 'random_preferences.pl'
Fix: Add import for `plunit` in 'random_preferences.pl'
Add basic tests for random_preferences/2
Add nondeterministic tests for random_preferences/2
Enable time measurement of failing goals

This is achieved by introducing try/1 predicate that always succeeds.
It is used in place of call/1 in call_repeatedly/2.
Rename call_repeatedly/2 to try_repeatedly

This reflects its new behavior introduced in last commit.
Rename tests block srp -> srp_2