diff --git a/src/alire-platform.ads b/src/alire-platform.ads
new file mode 100644
index 00000000..f46a0d4a
--- /dev/null
+++ b/src/alire-platform.ads
@@ -0,0 +1,13 @@
+package Alire.Platform with Preelaborate is
+
+   type Operating_Systems is (Unknown,
+                              Linux,
+                              Windows);
+
+   type Compilers is (Unknown,
+                      GNAT_GPL_2017,
+                      GNAT_FSF_7_2);
+
+   function Compiler return Compilers is (Unknown);
+
+end Alire.Platform;
diff --git a/src/alire-requisites.ads b/src/alire-requisites.ads
index 01cd0e96..ccf02047 100644
--- a/src/alire-requisites.ads
+++ b/src/alire-requisites.ads
@@ -26,9 +26,9 @@ package Alire.Requisites with Preelaborate is
       type Compatible_Property is new Property with private;
    package Property_Checker is
 
-      type Requisite is new Requisites.Requisite with null record;
+      type Requisite is abstract new Requisites.Requisite with null record;
 
-      function Is_Satisfied (R : Requisite; P : Compatible_Property) return Boolean;
+      function Is_Satisfied (R : Requisite; P : Compatible_Property) return Boolean is abstract;
       --  This is the important function to override by Requisite implementations
 
       --  The remainder methods are utilities that do not require modifications by the client.