Add derive2

resolution.py

@@ -95,6 +95,29 @@ def derive(clauses: Sequence[Clause]) -> Sequence[Clause]:
         for clause in merge_clauses(c1, c2)
     ]
 
+def derive2(kb1: Sequence[Clause], kb2: Sequence[Clause]) -> Sequence[Clause]:
+    """
+    All clauses which derive in one step from the combination of two knowledge bases
+
+    Each resulting clause is the combination of one clause from the left knowledge base
+    with exactly one clause from the right knowledge base.  Clauses from the same
+    knowledge base will not be combined.
+
+    >>> derive2([
+    ...     [IRNeg(IRProp('animal', [IRProp('Kim')]))],
+    ...     [IRNeg(IRProp('cat', [IRVar('x1')])), IRProp('animal', [IRVar('x1')])],
+    ... ], [
+    ...     [IRNeg(IRProp('dog', [IRVar('x0')])), IRProp('animal', [IRVar('x0')])],
+    ...     [IRProp('dog', [IRProp('Kim')])],
+    ... ])
+    [[¬dog(Kim())]]
+    """
+    return [
+        clause
+        for (c1, c2) in product(kb1, kb2)
+        for clause in merge_clauses(c1, c2)
+    ]
+
 if __name__ == '__main__':
 	import doctest
 	doctest.testmod()