Git Init with small 'isa' demo

Change-Id: I39ed4809035ac72e822934b063617bab1de86fa9

Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "arcs-ibis"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+crepe = "0.1.5"
+lazy_static = "1.4.0"

src/main.rs

@@ -0,0 +1,70 @@
+#[macro_use]
+extern crate lazy_static;
+
+use crepe::crepe;
+use std::collections::HashMap;
+
+type Ent = u32;
+
+struct Context {
+    last_ent: Ent,
+    names: HashMap<Ent, String>,
+}
+
+impl Context {
+    fn new() -> Self {
+        Self {
+            last_ent: 0,
+            names: HashMap::new(),
+        }
+    }
+
+    fn get_ent(&mut self) -> Ent {
+        self.last_ent += 1;
+        self.last_ent
+    }
+
+    fn name(&self, id: &Ent) -> &str {
+        // TODO: Check!
+        self.names.get(id).unwrap()
+    }
+
+    fn decl(&mut self, name: &str) -> Ent {
+        let new_id: Ent = self.get_ent();
+        // TODO: Check!
+        self.names.insert(new_id, name.to_string());
+        new_id
+    }
+}
+
+crepe! {
+    @input
+    struct IsAClaim(Ent, Ent);
+    @input
+    struct Exists(Ent);
+
+    @output
+    struct IsA(Ent, Ent);
+
+    IsA(x,x) <- Exists(x);
+    IsA(x,x) <- IsAClaim(x, _);
+    IsA(x,x) <- IsAClaim(_, x);
+    IsA(x, z) <- IsAClaim(x, y), IsA(y, z);
+}
+
+fn main() {
+    let mut ctx = Context::new();
+    let mut runtime = Crepe::new();
+
+    let socretes: Ent = ctx.decl("socretes");
+    let man: Ent = ctx.decl("man");
+    let mortal: Ent = ctx.decl("mortal");
+
+    runtime.extend(&[Exists(socretes), Exists(man), Exists(mortal)]);
+    runtime.extend(&[IsAClaim(socretes, man), IsAClaim(man, mortal)]);
+
+    let (reachable,) = &runtime.run();
+    for IsA(x, y) in reachable {
+        println!("{} is a {}", ctx.name(x), ctx.name(y));
+    }
+}