Add predicate-cardinality combinators

[Seddryck]

Summary

Add a family of predicate-cardinality combinators:

satisfies-exactly(count, p1, p2, ...)
satisfies-at-least(count, p1, p2, ...)
satisfies-at-most(count, p1, p2, ...)

Each predicate is evaluated against the same input value. The functions return a boolean according to how many supplied predicates are satisfied.

Semantics

• satisfies-exactly(n, ...) returns true iff exactly n predicates are satisfied.
• satisfies-at-least(n, ...) returns true iff at least n predicates are satisfied.
• satisfies-at-most(n, ...) returns true iff at most n predicates are satisfied.

count must be a non-negative integer.

Short-circuiting

The functions should stop evaluating predicates as soon as the final result is known.

Examples:

• satisfies-at-least(2, ...) can return true after the second satisfied predicate.
• satisfies-at-most(2, ...) can return false after the third satisfied predicate.
• satisfies-exactly(2, ...) can return false after the third satisfied predicate, or when too few predicates remain to reach two.

Relationship with existing logical combinators

These functions generalize several common logical concepts:

• any(...) is equivalent to satisfies-at-least(1, ...).
• none(...) is equivalent to satisfies-exactly(0, ...).
• an exactly-one(...) helper would merely be satisfies-exactly(1, ...), so no separate primitive is required.
• all(...) remains a useful named logical concept because its target cardinality depends on the number of supplied predicates.
• majority(...) remains a separate named combinator because its threshold also depends on the number of supplied predicates.

Requirements

• Accept any number of predicate arguments after count.
• Evaluate all predicate arguments against the same input value.
• Validate the count argument.
• Define behavior when no predicate arguments are supplied.
• Implement short-circuit evaluation.
• Add conformance tests and function metadata/documentation.