Add a passthrough leaper

src/lib.rs

@@ -29,7 +29,7 @@ pub use crate::{
         extend_with::ExtendWith,
         filter_anti::FilterAnti,
         filter_with::FilterWith,
-        filters::{PrefixFilter, ValueFilter},
+        filters::{passthrough, PrefixFilter, ValueFilter},
         Leaper, Leapers, RelationLeaper,
     },
     variable::Variable,

src/treefrog.rs

@@ -190,6 +190,25 @@ pub(crate) mod filters {
         }
     }
 
+    /// Returns a leaper that proposes a single copy of each tuple from the main input.
+    ///
+    /// Use this when you don't need any "extend" leapers in a join, only "filter"s. For example,
+    /// in the following datalog rule, all terms in the second and third predicate are bound in the
+    /// first one (the "main input" to our leapjoin).
+    ///
+    /// ```prolog
+    /// error(loan, point) :-
+    ///     origin_contains_loan_at(origin, loan, point), % main input
+    ///     origin_live_at(origin, point),
+    ///     loan_invalidated_at(loan, point).
+    /// ```
+    ///
+    /// Without a passthrough leaper, neither the filter for `origin_live_at` nor the one for
+    /// `loan_invalidated_at` would propose any tuples, and the leapjoin would panic at runtime.
+    pub fn passthrough<Tuple>() -> PrefixFilter<Tuple, fn(&Tuple) -> bool> {
+        PrefixFilter::from(|_| true)
+    }
+
     /// A treefrog leaper based on a predicate of prefix and value.
     /// Use like `ValueFilter::from(|tuple, value| ...)`. The closure
     /// should return true if `value` ought to be retained. The