@Test public void testValidity() {
Environment e=Environment.EMPTY;
e.validate();
e=Def.create(Symbol.intern("foo"),Constant.create(1)).compute(e);
e.validate();
e=Def.create(Symbol.intern("bar"),Lookup.create("foo")).compute(e);
e.validate();
e=Def.create(Symbol.intern("bar"),Lookup.create("baz")).compute(e);