as in:

  with Ada.Text_IO; use Ada.Text_IO;
  with System.Dim.Float_Mks; use System.Dim.Float_Mks;
  with System.Dim.Float_Mks_IO; use System.Dim.Float_Mks_IO;

    procedure Main is
       subtype Distance is System.Dim.Float_Mks.Length;
       subtype Area is System.Dim.Float_Mks.Area;

       D1 : constant Distance := 10.0*m;

       D2 : constant Distance := 20.0*m;

       -- D3 : constant Distance := D1 * D2;
       -- Does not compile. GNAT returns the following error:
       --   main.adb:13:08: dimensions mismatch in object declaration
       --   main.adb:13:37: expected dimension [L], found [L**2]

       A : constant Area := D1 * D2;
    begin
       -- print: Area A is  2.00000E+02 m**2
       Put ("Area A is ");
       Put (Item => A);
       Put_Line ("");
    end Main;