Add greeting example.

examples/greeting.flg

@@ -0,0 +1,30 @@
+(*-
+ * #%L
+ * Formulog
+ * %%
+ * Copyright (C) 2018 - 2019 President and Fellows of Harvard College
+ * %%
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * #L%
+ *)
+
+@edb rel entity(string)
+entity("Alice").
+entity("Bob").
+entity("World").
+
+rel greeting(string)
+greeting(Y) :-
+  entity(X),
+  some(M) = get_model([`#y[string] #= str_concat("Hello, ", X)`], none),
+  some(Y) = query_model(#y[string], M).