Initial data commit

Task/Abstract-type/Sidef/abstract-type.sidef

@@ -0,0 +1,17 @@
+class A {
+    # must be filled in by the class which will inherit it
+    method abstract() { die 'Unimplemented' };
+
+    # can be overridden in the class, but that's not mandatory
+    method concrete() { say '# 42' };
+}
+
+class SomeClass << A {
+    method abstract() {
+        say "# made concrete in class"
+    }
+}
+
+var obj = SomeClass.new;
+obj.abstract();   # made concrete in class
+obj.concrete();   # 42