Add initial prototype.

3-mid/physics/interface/source/private/box2d/box2d_physics.ads

@@ -0,0 +1,15 @@
+with
+     float_Math;
+
+package box2d_Physics
+--
+-- Provides an implementation of the physics interface using a binding to the Box2D C library.
+--
+is
+   pragma Pure;
+
+   package Math renames float_Math;
+
+   Error : exception;
+
+end box2d_Physics;