Don't crash on valid input (i.e named empty terms

wbnf/compile.go

@@ -127,7 +127,8 @@ func buildAtom(atom AtomNode) parser.Term {
 	case "term":
 		return buildTerm(atom.OneTerm())
 	}
-	panic("bad input")
+	// Must be the empty term '()'
+	return parser.Seq{}
 }
 
 func buildQuant(q QuantNode, term parser.Term) parser.Term {

wbnf/grammar_test.go

@@ -258,3 +258,11 @@ func TestCombo2(t *testing.T) {
 	assert.NoError(t, err)
 	log.Print(p.Grammar())
 }
+
+func TestEmptyNamedTerm(t *testing.T) {
+	t.Parallel()
+
+	p, err := Compile(`x -> rel=();`)
+	assert.NoError(t, err)
+	log.Print(p.Grammar())
+}