[MOD] trying old stuff on julia 1.2

11_DefiningTypes.jl

@@ -1,4 +1,4 @@
-abstract Shape
+abstract type Shape end
 
 type Circle <: Shape
     x::Float64

18_Interfaces.jl

@@ -0,0 +1,5 @@
+abstract type Expression end
+
+function evaluate(exp::Expression,ctx::Context)
+    error("Must implement evaluate", typeof(exp), ", ctx)")
+end