BASE <http://a.example/>

ABSTRACT <A>          /sA..........$/
                  AND
                      { <p> [0 1 2 3 4 5 6 7 8 9]+ }
ABSTRACT <B>          EXTENDS @<A> {}
                  AND /s.B.........$/
                  AND
                      { <p> [0 1   3 4 5 6 7 8 9]+ }
         <C>          /s..C........$/
                  AND
                      { <p> [0 1 2   4 5 6 7 8 9]+ }
         <D>          EXTENDS @<B> {}
                  AND @<C>
                  AND /s...D.......$/
                  AND
                      { <p> [0 1 2 3   5 6 7 8 9]+ }

# 0 1 2 3 4 5 6 7 8 9+                         
# 0 1   3 4 5 6 7 8 9+                         
#       -            +                         
# 0 1       5 6 7 8 9+